I've been a massive Pokemon fan since the games first dropped in the United States but I agree with you 100%
Yes Pokemon is insanely popular but 9 is definitely more than enough characters, not to forget all the poke ball Pokemon in the game as well, that's the main reason I'm not excited about Incineroar. I'm honestly kinda hoping Vergeben is wrong about that one lol.
Can you imagine Smash 20 years from now if this tread continues? They'll probably have to change the name to "Pokemon and Friends"
I think one new one per Smash isn't too much to ask for, but they did set an odd precedent in Smashes 1-3 by adding two for each (with Brawl "technically" adding 4). It wasn't really a problem in earlier games, as Squirtle, Ivysaur, Pichu, and Mewtwo (originally) were cut, but when putting them together in Ultimate, it looks a little silly. Ultimate should be allowed to add one more rep, but with Mewtwo, Pichu, and Pokemon Trainer's two other Pokemon coming back, it looks like it's added 4 characters already.
That said, I've never seen Pokemon Trainer as three individual characters. Each Pokemon has a fatal flaw screws it over in a few matchups and all three Pokemon should be used to make up for these flaws. Charizard has poor neutral since he's big and slower, Squirtle lacks KO power and is easy to launch, and Ivysaur is easier to launch and doesn't quite excel unless the matchup favors him. Sure, Charizard has the clout to warrant Smash inclusion, but Squirtle and Ivysaur don't really have a place in Smash unless conceived as part of a greater Pokemon Trainer moveset. Just an old Pokemon Trainer main weighing in...
TLDR: Pokemon looks like it has more reps than it should, even though it's been better balanced in the past.
Actually, I'd justify more Pokemon reps for the same reason that I'd justify more Fire Emblem ones- they're character based series with rotating protagonists. They don't keep the core cast the spotlight for every game like Mario and Kirby do- they need more characters to better represent the breadth of their series.