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.
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)
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.