But eventually the Smash roster will be too big, don't you think? The veterans aren't always going to come back if there's more games (and thus, too many characters). That's probably why Sakurai is insisting on working on the veterans than adding plenty of newcomers, because there's a limit for how many characters you can have on the roster and most likely doesn't want to cut too many of them just yet.
And I'm aware of 'star power' but that doesn't mean that they should be kept the same. Are you honestly saying you'd be happy if Marth stayed the same forever? With no changes whatsoever? Pretty much every veteran has changed between games, and that's to keep them interesting and not a v1.1 of the character.
Someone like Mario can still change for what we know. FLUDD could have the ability to float/recover like in the Sunshine games for all we know, and we haven't seen Link's side special which could be entirely different, and we still haven't seen Bowser or Samus in full detail so there's hope they'll be different. But nonetheless, Sakurai has always tweaked the veterans with new moves with every passing game so I'm confident he'll do something with Marth to not make him Brawl Marth v2.