Just keep poisoning that well. There is absolutely no need to call it an awful argument, or say the justification is bad, because you don't agree with it. Instead, you only need to justify your reasoning behind this decision. You just keep taking shots at overlevelling, as if it isn't a valid play style. Just because you don't like it, doesn't mean others agree with you. And clearly, since this has been brought up in the Discord server, some people kinda like overlevelling.
It's absolutely fine to balance a game however you want and expect people to place a certain way, but you don't want to force people to play a certain way. People have some choice in the game with the Pokemon selection and the IV/EV stuff, but how about giving them more choice? How about letting them overlevel because it's their play style. Pokemon is meant to be an RPG, and RPG games usually give players choice in how they play. You're just removing a choice because you don't like it. Overlevelling is a completely valid playstyle, whether you like it or not. If someone wants a level cap, let them determine that for themselves, or maybe, if it's possible, put it in as an option. I've seen people put some interesting options into their fan games before, I imagine a level cap is possible. Also, Emerald Kaizo solved this issue pretty well. You can do whatever you want the entire game until the end, then everyone is lvl 100. Everyone's at the cap and on an even playing field.
But yeah, it's your game, you can obviously do what you want. Just don't talk muk when you remove a certain playstyle and people that like that playstyle have questions or say something about it. Don't act as if your way is superior.