If you can come up with a better solution, you'll have an entire industry's worth of money coming your way. No one likes the kernel-mode anticheat stuff, but no one's come up with a better solution either. Cheaters suck.
Rootkit is defined by intent, not by capabilities.
If a user agrees to a kernel level anti-cheat, it's not a rootkit.
I'm certain most users don't know what they're agreeing to. It's sort of the same argument people make about Meta et. all spying on people. Well, it's not spying, because you agreed to the EULA.
Who reads the EULA? Nobody knows what they're agreeing to, ever. Even for computer-savvy individuals, do they know all of what the kernel-level anti-cheat does? Of course not. Even their consent isn't informed. For normal users, they don't know anything about anything.
Linux's inability to run specific anti-cheat solutions is a vendor support issue on the anti-cheat maker's part, because they don't care about your security, and they've managed to convince game developers that this practice is acceptable. It's not. Vote with your wallet.