The strategies will be better as you'll have three different fighters to choose for any given situation. That's why I love that Pokemon Trainer's back. I give up one move, for several. It's subjective, but Pokemon Trainer is better than Charizard in a variety of ways, and I was really disappointed when it was just Charizard in 4 (I loved rock smash, but it's a sacrifice I'm down for).
Being "better" doesn't matter. It's still being forced to use Pokemon you don't want to use.
And losing a move people actually want to use.
Solo Charizard is something people prefer. That's not a feasible option with Pokemon Trainer as is. They aren't looking for better here. They are looking for playing as one character. Which is impossible in Ultimate. Because I don't see a solo Pokemon option being available(especially since they'd have to make new Final Smashes and Down B's for all of them. Though I would be up for a clone of Charizard only using his 4 moveset. Would please a lot of people).
People are actually ignoring the argument they're making. Tiers are meaningless here. They want solo and only solo. It's that simple. Stamina was hardly the issue in the first place and that along with type effectiveness were just weaknesses of the Brawl design, not the only thing that made PT something people weren't interested in. They just want to separately play as each character, without transformations(especially forced) and not missing any moves.