I honestly don't think people who are a bit unsatisfied with the final base-game roster should be labeled as ungrateful. Even ignoring the leak altogether, newcomers such as Shadow and Isaac were hugely requested and Zelda hasn't received a new playable character besides the different iterations of Link in almost two decades. The lack of satisfaction stems from the apparent absurdity of some of the choices related to the roster.
I don't blame Sakurai for choosing to limit the number of base-game newcomers given how much effort developing this game has required, but choosing to make them Assist Trophies once again feels... bad. I (and many others, I'm sure) would have preferred them to be DLC instead of Assist Trophies. I don't know, Sakurai's choice doesn't make any sense to me, since it's a bad choice even from a financial standpoint (they would have sold like hotcakes).
Basically, I believe people would have been less salty if Isaac and Shadow hadn't been in the game at all (until a potential DLC announcement), since it would have left the door open for them to be added later. Right now, it looks like their fate is sealed, which is a huge shock considering the fact that a character like Piranha Plant was announced to be playable. Why have the Ballot if you're going to ignore some of the top choices?
Don't get me wrong, I have nothing against Piranha Plant himself, he's just the victim of bad timing and I would have been more than happy to see him included knowing that we could still get these highly-requested characters in the future. I think it's reasonable for some people to be slightly annoyed by the fact that Sakurai has chosen to make a generic enemy playable (completely generic, this time, unlike Bandana Dee) while relegating important protagonists/antagonists to Assist Trophy status once again. At the same time, I don't think comparing Piranha Plant to R.O.B., Mr. Game & Watch or Wii Fit Trainer is fair, since they are either the main characters of their games or, in R.O.B.'s case, a unique moment in Nintendo's history.
Basically, had very popular characters like Shadow and Isaac been part of the DLC season, this game could have been perfect for more people than it's gonna be in this timeline. That alone seems like a good enough reason to be disappointed today, despite Sakurai's work which we all respect. Tomorrow, we know that we'll still have the biggest game in the series, we know we're still gonna have 103 stages, countless characters and so on, but we also know what we will probably never have in this same extraordinary and once-in-a-lifetime "Everyone is Here!" game - the possibility to play some characters that have been highly requested for years.
