If we weren't coming up on the end of Smash Ultimate (I mean relatively speaking, these next 5 fighters are clearly going to take forever to actually release lol), I think I'd be more receptive to the whole arguments about expanding existing series, but it's hard to not look at Ultimate's achievements and not wish for them to push it as far as they can. There's still a LOT of series both first party and especially third party that aren't in Smash and would fit very naturally and make additional dream match-ups happen. Especially when we more or less know that we're objectively not going to see this many properties again. Third parties will be subject to some degree of a natural culling due to just the sheer logistics and cost (though they could return as DLC, just definitely not base game) and first parties are naturally going to lose some contenders as well. I don't really see Ice Climbers getting any amount of priority when so many cuts are on the table for example (and to be clear, I'm staunchly against cuts, I just acknowledge we're not getting Ultimate again).
That's in part why it's such a big deal to focus on adding new universes. There's little chance you're going to put Snake up against Cloud or even Ice Climbers up against Little Mac in the future outside of this more specific Smash context. Now, while yes, this can apply to individual fighters as well, but there's also no doubt that entire series tend to hold more sentimental value than individual characters, and getting a franchise in Smash makes an entirely unique dream match-up happen or gives your franchise one last chance to shine in the spotlight (Hell, Sakurai has done more to honor Metal Gear and Casltevania's legacy than Konami has in the past 5 years even beyond Nintendo's own neglected IPs). Characters from new franchises hold more weight because they aren't inherently just bringing themselves as I've said before, but the nostalgia of that franchise, the music you remembered playing in a battle, etc. That doesn't make them inherently better options, but it is important to note that extra value they bring to Smash and for fans.
I think it's also occasionally an underestimated idea of just "cramming franchises into Smash." I mean, first off, that has been Smash's legacy from the get go. It literally got famous because it crammed a ton of Nintendo franchises altogether in one game to begin with and has continued to evolve on the basis of doing exactly that and paying fan service to every franchise included along the way... But also, each franchise has huge fan bases that have either very little in the game or nothing at all, and some of these franchises are from titles that have sold 10, 20, 30 million plus copies across their life times and reached millions of people. Again, that doesn't make new franchises inherently better, but it's more of an explanation of the specific value of pushing new franchises for Smash and also why people would be so incredibly attracted to that idea. The more games you've played and the more you've immersed yourself in a variety of gaming landscapes and cultures, the more you recognize how many fans of different franchises exist across the medium and can individually recognize the enormous value of those franchises for yourself as well.
