Besides your argumentation that open source is less secure, a driver or program does not need to be in the Kernel to work with it. Does it? Kernel level anti cheat systems are available on Windows too, without being in the Windows Kernel. All it needs is a Kernel module to load it separately. Something like the Nvidia proprietary driver. I don't know if this would work for Anticheat.

Back to your point of open source and code around it. Well they code around the proprietary tools too. Reverse engineering stuff is possible. So your argumentation is a bit weak. Open Source means more people are looking into and its actually more secure and up to date (for common and actually developed drivers).

And you don't have to use it, if you don't like. How about letting people give options instead calling something they want or need being useless? It has a use and reason, so its by definition not useless. Instead using Windows, they could use Linux.