A foreword before I get to this blog is that this is not the place for anypony to debate his status. We all know what I believe and what others are inclined to believe, and nothing will change that. Also of note, if you choose to still think the same way about Sombra as you do after reading this blog, that's all right, you'll have your reasons and everyone is entitled to their own opinions so I can't force you to stray away from them, and attempting to do so would be against the brony way (which is common courtesy in any event), but I would like to take the time to address the two big complaints the fandom has had about the evil stair-obsessed Sauron-inspired unicorn king.
King Sombra is the least favorite villain in the show, possible exception of Sunset Shimmer (and she's quite popular now she's reformed). Only a few people seem to like him (like, he's my second favorite villain, and Screwball's favorite character), everyone else hates him. Why? Because they think him to be a lame villain due to lack of screentime, lines, and personality.
Well, that doesn't automatically make him a lame villain. It adds more mystery to him and makes him seem even more menacing in that we don't know as much about him. Also bits of his personality were revealed: He is extremely cruel and extremely intelligent.
Speaking of which, he is probably the most intelligent villain in the show because he planned everything ahead and set all those traps. Just think of them all for a minute. First, he hid the Crystal Heart in his castle where nopony would look, but also the fact that he set traps suggests that he foresaw the possibility of Celestia and Luna banishing him, since they'd only enter if he was gone. For the traps themselves, first you need to use dark magic to find his first staircase. On that note, you have to be a unicorn or alicorn to even do this. So you'd assume you have to use dark magic to activate the door as well, but instead it causes it to trap you in your worst nightmare, where you'll stay unless somepony else is there to snap you out of it. In the event a unicorn has been snapped out of her nightmare and used light magic to enter the next room, it is an endless stair that you need a gravity spell to reach the top. And once you get there, the Crystal Heart is protected by an alarm panel which will alert Sombra to their presence.
Also of note, he is arguably one of the most powerful villains in the show, rivalled only by Discord and Tirek. He is extremely powerful in his weakened shadow state, so imagine how powerful he must be in physical form. He is so intelligent and so powerful that he was able to outsmart and overpower Twilight Sparkle, who is known for her strength in both departments. And also the fact that he was able to take over the Crystal Empire on his own, which again serves as a testament to both his power, by overthrowing an empire and its princess (whom he presumably killed, and the comics have their take on that but are most likely non-canon due to an inconsistency regarding the cutie mark of said princess), and his intelligence, since he was somehow able to take the Crystal Heart without its defenses being triggered, because if they had, he wouldn't have lived to enslave the Crystal Ponies.
While Sombra did not appear much, the scenes in which he did appear are even more epic than Chrysalis's, and he is more dark and menacing than her, or even Lord Tirek (G1 Tirek on the other hand is equal to if not more dark and menacing than him). And the only villains who came as close to winning as him were Tirek and the Dazzlings.
Another thing to consider: Maybe the reason why Sombra didn't do more is because if he had, things would have gotten way too dark for this show, which further shows how dark, evil, powerful, and competent he is even when compared to Tirek, though Tirek comes close. Nightmare Moon and Discord screwed around and Chrysalis was easily distracted. Sombra does neither. The minute he finally entered the Crystal Empire, he would have killed the Mane 6, Shining Armor, and Cadance if he hadn't noticed Spike first, and most certainly would have killed Spike and taken the Crystal Heart if Cadance and Shining Armor didn't have a "Bob throw me" moment. He probably only didn't kill Shining Armor outside the barrier because he wasn't strong enough yet. It's even implied in later episodes that Sombra's intent upon his return was not to re-enslave the Crystal Empire, but to destroy it with his dark crystals.
Anypony who thinks Sombra is lame and has only seen The Crystal Empire once, I urge you to watch it again. A second time made me change my mind about disliking Just for Sidekicks and The Cutie Map.
When Queen Chrysalis fell in A Canterlot Wedding, people refused to believe she was dead because they liked her so much (which resulted in her return in the first arc of the comics, and there is no need for her status to be debated again as we all have our own headcanon on the matter). This, I can understand, and there was actually one villain in another show I liked so much I was disappointed to see her die (though in her case I wanted her reformed, which I'm guessing isn't what everyone else wants for their villains).
However, despite King Sombra being almost as universally disliked as Joffrey from Game of Thrones, Sombra's death was met with a very similar reaction to Chrysalis's apparent death: Disappointment from some, denial from others. Some still want to think he survived despite Meghan McCarthy confirming his death up front. Exact quote: "He's dead now, so who cares?"
Now, you don't have to like it, but maybe you can atleast accept it when you take this into consideration:
I think they felt the show needed at least one official villain death in order to measure up to G1 and Disney movies, both of which are kid-friendly and were not afraid to kill off some of their villains. Disney sometimes did this in brutal ways (most of which would go over the kids' heads) or else either magically destroy them or have them fall over a cliff, but the two villains to unambiguously die in G1, Tirek and Lavan, were magically destroyed (though Tirek's was a bit subtle, Lavan's was up front).
So, Friendship is Magic is on par with G1 and Disney movies in terms of audience range, and Hasbro and DHX were well aware of the fandom by now, so I think they kinda had to show that they weren't afraid to atleast magically destroy a villain. If it did think itself too much of a "kid show" to be able to do what G1 did twice, it probably wouldn't help the show's reputation, as people in the dark might be quick to judge it. With Nightmare Moon, the fandom hadn't existed yet and they couldn't even make her a full villain; with Discord, they hadn't quite gotten to the more extreme points yet and likely considered the possibility of reusing him; and with Chrysalis, I think Meghan intended to kill her but staged it in too subtle a way. They're afraid to kill off major villains now, but only because of the fans' above mentioned reaction to it (and besides, minor villain Arimaspi fell to his death in the Abysmal Abyss).
Now, we've got two villains (or rather four) that everyone likes a great deal more than Sombra or even Chrysalis, seeing as how they are many things everyone liked in Chrysalis. Lord Tirek (the G4 version), and the Dazzlings. Tirek's battles are more epic than Chrysalis's, the Dazzlings have a better villain song than her, and both have a lot of screentime and lines. And, both survive in the end, though they lose all their power. Tirek is imprisoned in Tartarus (where he'll probably be begging Cerberus to eat him after a few weeks of watching G3 in my headcanon), and the Dazzlings ran off and could potentially return for redemption. Personally I think reimprisoning Tirek was a little anticlimactic after such a huge Dragonball Z style battle (especially considering what happened to his G1 counterpart), but they have to please the majority of the fans and it was still my favorite episode.
Let's also not forget Discord, the one everyone liked because he was fun to have around, was reformed in order to tell more stories with his character, so he could appear in more episodes.
So, ask yourself this: If you had to choose one main villain to be killed off, just one, who would you choose - Tirek, the Dazzlings, or Sombra?
Also the fact that before the episode came out, everyone wanted him to die. Exact quote from an anon on October 13th 2012: This is the kind of villain that deserves to be turned to crystal and shattered, or having his lair collapse upon him as he gives his final,defying laugh toward his enemies. That's how bad guys go. They don't convert, the don't turn to stone, they are not blown away by a pity emotion. They go down in a storm of steel and fire whilst the victors look on with hope in their hearts that he is gone forever. There's no doubt it was done for us, but maybe they killed him because we specifically wanted them to, but more of you felt you didn't actually want it when it actually happened. Like when Renamon wraps her tail around Dr. Hax in those pictures (I don't understand it either). I've even heard complaints that they didn't kill Sunset Shimmer (before Rainbow Rocks of course), and unlike Sombra, she isn't the kind of villain who can get killed in family-friendly shows
And also, just coz he's dead doesn't mean he can't return - he was intended to appear in Octavia's dream in Slice of Life and have a cello battle with her, but was cut for time constraints, and unless I'm very much mistaken, he will appear in an altered timeline in the Season 5 finale (as I think Starlight Glimmer will time travel and stop Rainbow Dash from doing her Sonic Rainboom, which will prevent the Mane 6 from getting their cutie marks, which in turn will prevent them from defeating the villains, thus Nightmare Moon, Discord, Chrysalis, Sombra, and Tirek will be in power until the timeline is set right)