I'd argue he's more than possible for base roster though. Kingdom Hearts was as popular then as it is today. It's always been a pretty popular series, and Sora likely did well on the ballot because as I said before, the entire Kingdom Hearts community rallied around him for Smash. And by "community," I mean the entire Kingdom Hearts subreddit, fan forums, youtubers with over 10,000 subscribers making videos and tweeting telling their viewers to vote for Sora, Smash fans who just like Sora, and casual players of both games. I wouldn't be surprised if Sora ranked top 25 worldwide, if not top 15, or even top 10 in the ballot. People have wanted Sora as early as in Brawl, as evidenced by his showing up on Sakurai's Brawl Poll. Pre and Post-Sm4sh people asked for him, and people are still asking for him. Heck, there's currently a post on the Kingdom Hearts reddit with 1300 upvotes about Sora being in Smash (
link here).
Dude's more likely than a lot of people think.