The theory is that raising the HP of single prize Pokémon while freezing everything else is returning the game to generation 3 balancing.
And if you get it wrong you end up with completely uncontrolled power creep across the board for years to come.
The point of my post wasn't to take "balance" the way you have, but to argue for a reduction of HP and HP caps across the board, not an
increase. In terms of how you handle that given it'd be a step down from what came before;
- Hard format rotation to ensure the viability of the TCG as a long-term game. This would probably be unpopular, so the next best thing;
- Print more attacks that are specifically punitive against Pokémon V/VMAX/VSTAR but are not necessarily punitive against Pokémon ex. e.g. "If The Defending Pokémon is a Pokémon V, this attack does [some extra amount of damage]", various Trainer cards that punish players for having Pokémon V/etc in play. Then, once you've cleared the old format's multiprizers out, go after the multi-prizers of the current format. When multiprize cards originally debuted, there were a ton of cards with punitive "If the Defending Pokémon is Pokémon-ex..." attacks so that running Pokémon-ex really leaned into the entire idea of cost/benefit. People also forget multiprizers left the format for basically an entire generation, too, and there was an extended period of time where Lv.X cards were clunkier to get into play than Pokémon-ex (several Lv.X were
de facto Stage 4 cards).
Generation 5, like it was a mess for the video games, was also a mess for the TCG, since it brought back multiprize cards in such a lousy, hamfisted way, and seems to have baked in the contemporary thinking that the only way to fix the game is to keep making numbers go up. Gen 3 (and Gen 4) TCG game balance is entirely different from what you are proposing.