Tim Sweeney (of Epic/Unreal/Fortnite) is not a reliable narrator, but he's on record saying:
> Valve has done an amazing job there; I wish they would get to tens of millions of users, at which point it would actually make sense to support it.
Windows has never been my primary system, but there are times when I've had one because I wanted to do something (long ago 3D modeling, now PCVR) that wasn't supported on my main systems. If I could do that just as easily in e.g. SteamOS, Windows would once again exit my life.
The actual image that gets shipped on the Steam Deck is available in bits and pieces as git tags in Valve's clandestine repos, but isn't yet being distributed for other devices. That is changing this spring, when they start supporting other vendors.