My theory for a bit now has been that Valve is playing the long game in trying to make SteamOS a mainstay gaming platform as an alternative to Windows, and that the hardware products are essentially a way of breaking into that market. Even a few years ago, the idea of a custom Linux distro based on Arch Linux with both a built-in full desktop mode and a lower-powered gaming mode that you could switch between on a handheld device would have sounded kind of crazy, but now we're at the point where it's fully supported on more than one vendor's hardware. This seems like it could be a similar play in the traditional desktop space; if they can prove that the concept is viable, maybe other vendors will come out with similar products that come with SteamOS by default. All of this insulates them from having to worry about the long-term sustainability of making money from game sales on Windows, and if it works out, they wouldn't even necessarily have to continue making hardware indefinitely.
I don't pretend to have any insight into whether this theory is correct beyond that it seems to track with what they've been doing lately, or any expertise to make claims about whether it will work or not. In a lot of ways, this might just be a projection of my desires as a gamer who enjoys not having had to boot into Windows to play something for quite a few years at this point. I do hope that maybe they're just crazy enough to not only try this, but pull it off though!