If I am not mistaken, you /can/ have that be a thing, as I am fairly sure the actual SteamOS is open source and Valve would /probably?/ allow that to come stock on a potential alternate handheld…

But kind of their whole thing is it is optimized for the hardware set up they are using.

So… you could theoretically have an or multiple competing Not Valve but Yes SteamOS Decks, but from the standpoint of the economics of building a market viable gaming machine, Valve would still probably have a serious edge in that market for a while.