We keep hearing that the next generation of Xbox hardware will be more of an open platform rather than a walled garden similar to the way Valve operates. They make their own niche hardware for whoever wants it but you can still access the exact same storefront/games on competing hardware. That's probably why it keeps getting alluded to that there will a traditional premium console and a handheld. Whether the devices uses a proprietary OS or an entirely different version of Windows, like what Valve is doing with SteamOS, who knows.
Valve tried this approach a decade ago with Steam Machines, but we all know how that went, and they seem to have a good spot with the handheld PC market. Xbox can't just go full third party entirely. People have built up over 15 years worth of a digital library of games, achievements/gamerscore, etc. that MS can't just drop, and I'm sure they know that.
There's also a matter of how Microsoft handles their other hardware businesses. Surface barely competes in the laptop space. It even has a smaller marketshare in laptops than Xbox has in the console space, and yet, Microsoft continues to make that hardware, even though you get can get the same version of Windows on dozens of other types of laptops. Satya Nadella has even talked about how he regrets leaving the mobile phone space, and we know how horribly Windows Phone performed.
So Microsoft when it comes to their hardware, seem completely content with it being a niche part of their overall business. So I kind of expect Xbox, especially with how much money has been invested in the division in recent years, to start to operate in the same way Microsoft handles the rest of their hardware business. It's there for whoever wants it. There does seem to be a dedicated Xbox userbase when it comes to hardware that will always buy Xbox hardware, and Microsoft seems to be happy with that. Microsoft is a software and services company first, and that's what they want to focus on.