Earlier quoted context omitted.
The last remaining roadblock is kernel level anti-cheat frameworks. Pretty horrible technology, and unfortunately a good majority of the gaming industry by revenue relies on it.
Competent cheat makers don't have much difficulty in defeating in-kernel anticheats on Windows. With the amount of insight and control available on Linux anticheat makers stand little chance. The best Valve could do is offer a special locked down kernel with perhaps some anticheat capabilities and lock down the hardware with attestation. If they offer the sources and do verified builds it might even be accepted by so…
Also, for more casual play, don't players have rankings so that you play with others about your level? Cheaters would alll end up just playing with other cheaters in that case, wouldn't they?