This. Sakurai brought Kid Icarus back from obscurity and made a game so chock-full of content and memorable that he wouldn't have to make another game. Which is just as well, considering he has no plans to make a sequel.
Smash 4 is, as well as anything else, Kid Icarus's legacy testament. Sakurai didn't deliberately include loads and loads of content from the game to stroke his ego, he did it because it was convenient and he knows people are fans of the game. That it will ensure Kid Icarus continues for a long time to come is just a bonus feature.
Long story short, Kid Icarus pretty much needs Smash to carry on. Unless another developer decides to pick up Pit and friends - and Sakurai has made it clear he's hesitant to pass on the reins to somebody else - the series is pretty much finished. In a game that has over fifty characters, why would one extra character - that, it must be stressed, took about a month of dev time, at most, to create - be such a big deal? It may seem like Dark Pit was included just so Sakurai could pat himself on the back for how well he did for himself, but it's also a tribute to Kid Icarus fans. It's the way Kid Icarus will carry on, regardless of whether or not Sakurai or another developer intends on formally continuing the series: the conversation between Pit and Pittoo in their Palutena's Guidance speech proves that Sakurai is enabling his characters to tell their story even after Uprising's credits have finished rolling.
Is it bias? Without a doubt. But bias is not necessarily a bad thing. Sakurai is biased towards his series because it's his series. He can't do anything with, say, F-Zero or Mother because they aren't his series. He has no reason to or intent to tell those characters' stories, because he's not developing those games or telling those stories in the first place. Kid Icarus relies on Sakurai, though, and he has an opportunity to do something with that series to please fans and show them that there's still something to look forward to. I may not be a big fan of Pittoo as a character - I prefer Pit myself - but I'm very, very happy that he won't be ditched or tossed into the trash heap anytime soon.
