As we mentioned just seven days ago, the Asus Rog Ally will be able to use an official version of SteamOS. Valve has confirmed it in an interview with The Verge, who contacted the company to ask, especially, about that point in the SteamOS 3.6.9 beta notes that described support for extra Rog Ally buttons. The company's response was affirmative, and it also assures that it is only the first step.
As mentioned, Valve It does with the Steam Deck the same thing that Microsoft and Sony do with the Xbox and PlayStation respectively: it sells a device at cost price and then makes money with the games. Currently there is no "Steam+" or "Steam Pass" or anything similar to play with by subscription, so it makes no sense to think about user loyalty. The more people who can play, the better. It's something I would also do if I were Microsoft or Sony, but...
SteamOS will come to more consoles
According to the designer of the Steam Deck, SteamOS will come to third-party devices, not just the Ally. The Asus console or handheld PC will only be the first, but support will be extended. It is almost certain that it will reach the MSI Claw and Lenovo Legion GO, which are among the most popular, but I would also put the Orange Pi Neo into the equation, which will come by default with Manjaro.
Personally and knowing Valve's plans, It cannot be ruled out that they will release an ISO for PC either., or that what they release for the Ally and other handheld computers is a generic image with official support for some handhelds and compatible with hardware in general, such as the Linux kernel. In fact, timid rumors are circulating that Valve could also launch a version of the Deck but only a computer, which would be a slightly more powerful tower or box. Parole, soltanto parole.
Better performance?
The world of handheld computers is being very profitable, and there are even YouTube channels that seem to make money just talking about that and related topics. If we pay attention to what they publish, the Asus Rog Ally gets more FPS to Bazzite than with Windows. If I had a console with the Microsoft system, I think the best thing would be to do the opposite of what I do with the Steam Deck: Leave Windows installed and use the system that came with it almost always, but I would create an SSD with SteamOS if I want a little more performance.
In any case, in the future you will be able to choose what to do.