Well, i think it was inevitable, tbh.
There's not that many Nintendo characters left that would make the average consumer hyped
Man, I think we have to stop speaking on behalf of the "average consumer". The average consumer isn't a diehard Nintendo historian like us, but they're not all gluttonous hype monsters either. If the average consumer is only going to buy Smash Bros for massive third party crossover appeal, then why did the other games sell so well? At the end of the day, the most average of average consumers is buying the game because it's fun to play with friends, it has Mario on the cover, and/or because they've played previous games in the series and want to try out the new one. Neither of these extremist, polar opposite stances of wanting niche, thirty year old Nintendo characters or refusing to buy the game unless Master Chief is in it are average.
Ultimate sold
at least 12 million copies within the first few months, with only Simon Belmont as a new third party character and the promise of Joker a bit later. Its central hype statement rode on Solid Snake and a handful of decidedly less hype Nintendo characters. The biggest newcomers targeted diehard fans and ballot voters (Ridley, K. Rool), with a handful of mascots (Inkling, Isabelle) and Pokemon (Incineroar) that might play into the ideals of a more casual fan. DLC certainly kept the train rolling, but rode on the reputation that Smash had already established for itself. The fact that people were out of their seats excited for the likes of Ice Climbers, Young Link and Wolf should say everything about Smash's ability to liken audiences to characters they didn't know they would love.
But the initial statement is there aren't many Nintendo characters left to please this stock consumer. How is that the case, though? The Switch is one of the best selling consoles of all time, and the top 20-ish best selling games on the console were all Nintendo series. So it's a safe assumption that the average Switch owner takes a liking to at least a handful of Nintendo games. Between Mario, Zelda, Pokemon, Animal Crossing and Splatoon are at least a dozen great candidates that will be familiar to a solid chunk of the Switch's install base.
If we add a handful of those and then a couple recognizable third party characters as per usual, we are set. If the roster means that much to this player, who I guess doesn't play that many video games to begin with, a few characters that they recognize from games they like will be all that's necessary to get them interested. And that could vary across any demographic.
Young Nintendo fan - They'll buy the game with Mario, Link and Pikachu in it. This might be their very first Smash game.
Casual Nintendo fan - Maybe plays with friends and picks up on a couple characters they enjoy. Playing as Waluigi or Tom Nook would probably make them laugh. Might even grow to love a character they're unfamiliar with.
Nostalgic Nintendo fan - Could be a range of at least 20-30 years, but a number of throwback characters may appeal to their sensibilities. "They added Skull Kid? I loved Majora's Mask when I was a kid man!"
And what happened to the beauty of discovering brand new characters? If many players are asking "who is this character", they will eventually learn. Smash did a great deal toward introducing me to what would become some of my favorite series. I think speaking on behalf of an average consumer who has no interest in this sense of discovery, doesn't play enough games on the console they own to care about any Nintendo characters, cares exclusively about faces on the roster and not about having fun with their friends is doing this imaginary person a disservice. Whenever people say things like this, it's starting to sound a bit like we're talking down to this audience who doesn't know any better and needs to be spoon fed A-listers constantly. Believe it or not, people cared about this game well before they knew Sonic, Cloud or Steve would be in it and I don't think it's any different today.
All of that to say, it's not like we won't get more third party characters. I'm certain it will be a bulk of the DLC, and at least a good two or three new series will probably be on the table for base game. The consumer who only cares about Smash because they want to participate in a mindblowing, record breaking crossover hype cycle just has to wait a year or two.