It's a bit of a conundrum. If you have any kind of in-game reward at all for gaining XP, then you're incentivizing players to level up as quickly as possible (intentionally or not). But if you don't do that, then you've given the player nothing to indicate any sense of progress or achievement.
Or, put another way, players min/max because that's how you "win" by the game's own rules. The only way to "fix" that is to make an "unwinnable" game.
I'm not saying I have a solution, of course. There are ways to mitigate it, certainly, but I haven't seen a single game that can't be min/maxed somehow. In fact, most players seem to think that's precisely what "git good" means. C'est la vie.
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