Yeah, but a slightly more informed player could definitely take a Blastoise all the way. And speedrunners are of course some of the most informed players around.
There's a quite a few speedruns that do switch mains throughout the run but none that really use a party of Pokemon(for battling) as far as I know. Interestingly the Pokemon Shield run, which is quite different from Sword's, was one of them. Not sure if that's still true today though.
Sw/Sh runs now are dive into Crown Tundra, do the Dynamax Adventure and use the Level 60 you get in the first battle to blitz through the game, it's a fair bit different. I think Alolan Raichu is considered one of the best ones to get.
But the more informed part is key because if there wasn't a reason to become more informed, why would you. I mainly learnt most of the battling system and matchups from Pokemon Stadium than I ever did from Red or Blue just because of the same levels there. Closer battles let you get more fun out of the game for some people (Obviously not all, some people do just get satisfaction out of Getting Big Numbers and crushing all that stand before you) and that's become ever harder with the new EXP changes.
I'm planning to do a big write up post on my thoughts of this, but one thing I would really like to be seen done, and this goes for all RPGs not just pokemon, is use something like The World Ends With You Level Capping system.
You have a max level that you increase with EXP as you play the game, but you also have the option to lower the level of your characters whenever you wish to make things more challenging with a slider. With the incentive being for every level lower than your maximum that you fight at, the greater chances you get at getting rare pin drops from the enemies.
For a pokemon equivalent, you can make it something similar where if you chose to set your team to a lower level than they currently are, you can get a post-game currency rewards like Bottle Caps or Armorite Ore or Battle Points. Something that won't matter to a person that just wants to beat the game as quick as possible, but would be tempting for someone that wants to go into post game battles and such.
And it also just allows the player to make things a bit harder on themselves if they wish to if they are finding the games a bit too easy for their liking at the moment. Something which the more mature playerbase would probably appreciate.