The tools for owning your Linux OS are strong enough that anti-cheat is pointless because they're just broken all the time and nobody wants a linux box they can't control at all.
I think most people who buy Steam Decks don’t care whatsoever about Linux and would be perfectly fine with not having control over it as long as all their games worked.
I think Steam Decks wouldn't ever have existed without Linux enthusiasts as early adopters of Steam Decks and the few previous iterations of Steam + Linux either playing games on their own machines or on the previous iteration of a Steam Linux computer. If at any point it was all tied up with DRM and that complete loss of control required for anti-cheat it would have just died and not be seen again.
The only way it changes course is an enormous rug pull that removes most of the differentiation between PC and Console gaming and you end up with Steam as a dying product unable to compete with either other modes of PC gaming or the dominant console players. (Sadly that's basically what I expect when gaben retires)
The differentiation of the Steam Deck is the game ecosystem, ability to play your existing PC game library on the go, and low game costs compared to consoles during the frequent sales.
I don't think Linux is a differentiator for the Steam Deck. It's obviously essential as a technical foundation though, similar to how it’s essential to Android phones.
But locking it down with DRM won't affect gamer interest in the platform as long as the games are still cheap, plentiful, and run well.
I can imagine a world where you still have full control most of the time, but when you open a multi player game the system reboots on a clean / verified OS image. Then when you quit it can reboot in to the OS with all of your mods and customisation on.
