As far as “appealing to their fanbase” I think it’s understated just how many things have been covered with 1st party representation in the base game.
We have the main protagonist and main villain from several of the core franchises. We have over 10 Mario characters and 10 Pokémon characters. We have Nintendo characters new and old, highly requested and unexpected.
We even have Banjo-Kazooie, a character who characterizes a time many look back at fondly, and is one of the most hype inclusions from the viewpoint of “adding a character for first party fans”.
Now, I’m not going to say anybody should just “be fine with what they have”. That’s just rude. For context, however, we’ve got folks who are eatin good...dozens of quality Nintendo characters to choose from, stages, and music. Now, the highest requested Nintendo characters are lower in importance either as individuals or within the series they are in. While they are great characters, we’re down to picks like Dixie Kong, Waluigi, or Bandanna Waddle Dee. Dixie has been a protagonist in 2 games, one several years older than the other, while the other two are consistently supporting characters in series that have plenty of content in Smash already. You have characters such as these lines up with characters outside of Nintendo who are the lead of their respective games or series.
This is not to say they wouldn’t be quality additions, or that anybody is silly for wanting them.
However, the line of thinking that “1st party fans need to have a bone thrown to them” implies that:
1) Nintendo fans can only be pleased with Nintendo characters
2) Any 3rd party character satisfies fans who want specific 3rd party characters
Steve is cool, but does he appeal to fans of Monster Hunter or Devil May Cry? He could, but there are folks who specifically want their characters, who have no playable content or little content in the game.
Certainly, the same logic applies to folks who want Nintendo characters, but those folks might want Dixie and have 3 Donkey Kong characters, while somebody who wants 2B has...nothing.
At least, there’s where I can see the difference in appeal for fans, and the appeal for Nintendo to branch out to new brands. There is plenty of content from their own IP that it may be considered desirable on their end to continue collaborating.
So basically, I’m not telling Waluigi, Dixie, and BWD fans to shut up and accept what they have. Keep supporting your characters! However, just keep in mind where we currently are at and why many folks are more interested in 3rd parties. This doesn’t necessarily apply to myself, as I would likely be pleased with anything, but I advise folks to keep that in mind.
Frankly, if it makes anybody feel better I believe this is part of why some folks would prefer characters from new 3rd parties as opposed to those we already have. Though, most 3rd party rosters are already covered in non-playable forms. Shadow is an AT, Knuckles is an AT and costume, Tails is a costume, Zero is an AT, several Mega Man costumes, Dr. Wily is an AT, Yellow Devil Boss, etc.