Oh, I didn't want to critique Anti-cheat for existing and not properly working (yet) in Proton.

It's software that tries to validate the environment the game is running in and is suddenly confronted with "wait, thats not a Windows system at all". And as you already argued, just ditching Anti-cheat is not an option.

So they either ignore Linux/Proton altogether or have to start actually implementing anti-cheat for a Linux/Proton environment. Even if they go with the second option (as e.g. EAC is, iirc), it simply takes time to implement.