> - Valve has a bad track record for committing to their own hardware.

They actually have a pretty good track record when it comes to long term support of their hardware. Steam Link and Steam Controller have been getting new features and updates even after becoming unavailable to buy, and from what I heard Index owners can't really complain either. You may be thinking about Steam Machines, but those weren't actually Valve's hardware at all.

> I'm guessing lots of people will install windows.

...and they'll quickly go back to SteamOS, as running Windows on a device like that will come with quite a lot of suffering.