Do you think the Champions treat their Pokémon like Paul did? I don't think Cynthia was fond of his methods. In the games, Lance criticizes Silver for not caring about his Pokémon.
Cynthia told him to put aside his anger/disappointment after losing and making sure he goes to the Pokemon Center to treat his Pokemon before going back to training. She actually praises how well he trained his Torterra.
I would agree with that if Paul didn't break the number one rule of the Pokemon universe: that Pokemon are friends and companions, and that it's wrong to only care about how strong they are.
Paul showed that a trainer doesn't have to best buddies with Pokemon. Paul and Torterra are like Bill Belichick and Tom Brady of the New England Patriots; very close, built out of mutual respect from countless battles, but maybe not best friends like Ash and Pikachu.
All of his Pokemon are battle junkies who want to become stronger, and they all respect him, and were disappointed when he lost to Ash at the league.
The Pokemon anime can't do subtle. The writers simply changed Paul's character on the run, and hoped that the viewers would forget about how cruel and abusive he was. All in order to spew some nonsense about "different training methods".
I agree that the Pokemon fandom has a tendency to "over-analyze" the anime to justify the writing, especially when it's not trying to be subtle or deep.
But I don't know if I'd agree that the writers just "forgot" and changed his character. It's more of intention vs execution. They probably
wanted to show character development, with Paul changing after getting humbled by Cynthia and Brandon, and slowly appreciating Ash's methods, especially after he found out Ash beat the Battle Frontier, as well as gaining respect towards Ash for having the cajones to challenge him to a full battle, give the best fight he could, lose but still keep going, and obviously for continuing to train Chimchar and eventually master Blaze. Whether or not it was done in a compelling or rational way might be a different story.
Too bad people like AJ, Gary or Alain add to the diversity in a much more nuanced and tolerable way.
Those are all different characters than Paul.