Doesn't take reading past my first sentence of the post you quoted to see I didn't ascribe their inclusion solely to surprise.
And you entirely still ignored that "major player" applies to everyone but PP. So yes, it's not the same situation like you keeping saying it is. He never went back on his word because his word was never changed. They're all surprise characters at the end of the day. PP just had a far more unique reason for being chosen. And one of those is "not a hero or major player", which, let's be real, applies to tons of plausible surprise options we could get right now. So why would I look at tons of oddball characters who don't fit PP's reason for inclusion when they never stood a chance for that spot? The closest is someone like Waluigi, and as I said, he might already apply enough to "major player" as he hard starts off the plot of one game(spin-offs and mainline are hardly important to Mario anyway as a franchise, as they're all treated pretty much with the same core importance. Big games that sell and make it a notable franchise. This might apply for other series, just not Mario).
Besides, those surprise characters from before weren't "heroes" either, so the only change is, which isn't going back on his word, mind you(I don't know why you brought it up as it has no application in any way to the situation), is to have a non-major character added, especially as he doesn't want to see nothing but heroes and major characters(which PP is the only one that can't really apply, even loosely. Jigglypuff is kind of close to the same thought, but it starred in various episodes, so takes a "major role" for all intents and purposes. Though to be fair, it can also be in a similar position, but is just way more popular in general anyway, making PP still stand out mainly for being iconic, yet not a popular character).
At the end of the day, Sakurai doesn't just go for the same kind of category more and more. He adds pieces to make the category feel more unique and richer. That is still not a case of "going back on his word"(not that he doesn't change his mind, it's just not one of those cases).
tl;dr PP is a surprise character who has new criteria that doesn't apply to most surprise options nor does it apply to the previous surprise characters. Simple as that.
Lol ah words.
So how would ppl feel if Geno were an Assist? Is there a pattern for characters going from mii costume to Assist? Or being simultaneously an Assist and a costume?
That's better than just a costume(or deluxe). Though I'd take both anyway. Geno's awesome and seeing him in proper gameplay is great. Playable is the best option, but "fully showing his unique abilities in gameplay" is the second best thing.