The precise difference between 10 and 12 newcomers we clearly wouldn't know, and I mean 10 is within the +/- of my expectation, albeit on the lowest end. I'm basing this primarily on previous non-EiH Smash games, with some reservation due to having more vets than ever to potentially retain (albeit with a large number of them probably being lower priority than most of the newcomers).
But, regardless, generally I think it's better for them to err on the side of more newcomers than less. Because of, again, the circumstances of (1) needing to offset the fact that there will be subtractions to the roster and (2) needing, frankly, a lot of slots to promote more of their franchises and increase newcomer diversity, hence foster the overall appeal of the new roster.
I agree newcomers are essential, but I think the point of diminishing returns on the commercial effectiveness to character count happens earlier than you. Because even with fewer characters, I don't think this game is going to have any problem selling - it's Smash Bros - and therefore I think the powers at be will decide on the course in which base, which is a fixed price regardless of character count, will receive more newcomers than Ultimate but still towards the smaller end, and focus just as heavily on the subsequent rollout where you'll make more if you offer more. I think the establishing of a permanent Smash team that won't shrink after launch also shows them looking intently at a more ongoing model.
And I wouldn't base estimates on non-EiH Smash games, because those happened in an era either before Smash games were planned to have DLC when they started (or at all), in a time where receiving substantial ongoing updates was less commonplace, especially for Nintendo games.
But also, given how the landscape has changed, people now know that the characters they get in base aren't going to be the end... they know if they buy Smash they're going to get a few more years' worth of additions - so they go into the purchase with a valid expectation of more to come. Even knowing they will have to pay on top of what they've already paid. People didn't buy SF6 thinking this roster is all there's going to be.
I think you may be underselling the importance of extending on existing first party franchises. Like, for example, simply bringing Olimar back or even giving him a Pikmin 4 protag alt is not at all the same in generating and maintaining excitement about Pikmin as, say, introducing a fully brand-new Oatchi moveset. Bringing back Link in his BotW garb is not celebrating and promoting the Era of the Wild like revealing Rauru to show off all the fancy ToTK mechanics would be. Splatoon and Animal Crossing would begin to look horribly underrepresented relative to their current Switch and likely Switch 2 presences if they didn't get anything new. And so on and so on.
I think you may be overselling the importance of Nintendo characters in Smash translating to palpable sales of their series. With your examples of Pikmin and Zelda, those series have remained fairly static since Brawl in terms of fighters, yet flourished as of recently, commercially. Or look at Kirby, who is in a very similar boat.
These also show that, though it happens with the promo series and occasionally with others, Smash is also perfectly content to not give series characters just to give them another character.
Pokemon didn't get a Gen 8 or 9 character, and yet those are two of the highest selling gens. Likewise, Xenoblade's commercial peak was a game that at the time didn't have roster representation.
Fire Emblem got localized in part due to Smash, but then continued to perform worse and worse, despite growing in Smash prominence, only recovering based on a game that wasn't yet represented in the series. Kid Icarus on the 3DS didn't sell any better than Kid Icarus on the NES. I'm sure some of Uprising's sales were owing to Smash, but... I don't think it sold quite what Nintendo was hoping for. Star Fox kept getting characters as the series continued to sell worse title over title.
If you look at these examples, clearly it's the system's install base that matters far more.
Plus if you had even a limited handful of new first-party characters some of the series you mentioned
would be likely to expand. It's not going to just be like... Waluigi, Ring Fit and a Pokemon.
In addition to that, the DLC literally can't do them any good unless they successfully sell the base game. The better the base game does, the better the DLC does. Because the people who buy the DLC are a proper subset of those who buy the base game. The number of people who skip out on base initially but then are convinced to pay the price they wouldn't pay before plus more, just to get a few DLC characters they're particularly excited about, is rather few.
But thinking it's going to be hard to sell the base game is kinda ridiculous. That thing will move millions upon millions just by being Smash, having its staple faces, and throwing in
some new characters of appeal. People act like Smash is lucky to sell, like if they don't hit x character count the game will bomb... but this is a
very mainstream series. The primary indicator of its sales potential is again, the install base of the console over anything else.
I mean, they cut the Pokedex basically in half. And sure, some people were very mad, but... look at the numbers. These huge series aren't as fragile as they're cast around here. The next Mario Kart will undoubtably have much less content at launch than how much MK8D has chalked up over time.
Also, DLC does sell the base game. It's not where the bulk of sales come from, but clearly the goal with a lot of those inclusions is to lure in other fanbases towards Smash. That's why we get a disparate selection of often non-Nintendo-oriented third-parties of global appeal. They're purposefully casting a wide net.
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