To spend slots on characters who are always going to have a good chance (like your first parties) instead of those who will never have this chance again (because they'll be struggling to get Cloud and Pac-man back next time) is the textbook definition of a wasted opportunity. That's why it's mostly Three Houses, Astral Chain, and Sword/Shield who enter the conversation from the first-party side anymore - they don't have any guarantee that they'll still be likely newcomers in the future, especially if/when a newer game supplants them.
There are two fundamental flaws with that, methinks:
One: if these characters "will always have a good chance"... how come they're not in the game yet?
We feel like they have a good chance, but in the end we have no idea how Sakurai thinks. Some of these "have had good chances" for like 20 years now - because ironically enough they got passed over by picks from more recent games or more wanted fan choices.
Two: I dunno about SwSh, but I don't see Three Houses or Astral Chain going anywhere even if they don't get a rep in Ultimate. The former is one of the best selling FE games to date and the latter had a great response for a new IP. I can't see Nintendo not adding them later down the line just because they're "not the shiny new thing" anymore. Good games stand out on their own. I feel like if we have to argue that game X
has to be picked now because they don't have a chance later, then there's something wrong with either that game or the way the characters are being picked.
I don't think there is such a thing as a wasted opportunity in the end. Sure, I personally care more for third parties at this point, but... the reason we're getting more DLC is to make people happy. So unless we get something like Bubsy or Miner Ultra Adventures, it's hard to refer to anything as a waste, really.