Play Anywhere is awesome, buy once and have it on multiple platforms? Great.
Like I said as well, the biggest issue with the future Xbox/PC hybrid is not tech, it is licenses, it is that a license to play on Xbox doesn't give you a license to play on PC, but Play Anywhere does. So if Xbox can get a significant chunk of the average gamers library to be Play Anywhere before they make this switch to Xbox/PC hybrid then that would be amazing. So their focus from now on should absolutely be Play Anywhere until the switch happens, I imagine some sort of Xbox 360/Xbox One emulation will come to Windows PC as well in the future for those that aren't native.
Honestly I think Play Anywhere could go a long way to alleviating peoples concerns about the future of Xbox hardware too because either way their library would be safe, whether Xbox still made hardware or not, it would still be accessible via PC, Handheld, etc. The Xbox ecosystem they love would still be available. Their profiles, achievements, the UI, their library, etc.
I think they will probably need to try to aim for at least like 70-80% of the average gamers library being Play Anywhere before the switch to this Xbox/PC hybrid so that when the switch does happen people won't need to rebuy large parts of their library because this Xbox/PC hybrid will have a separate license agreement with publishers than the Xbox currently does as it will likely be running a Windows hybrid with an Xbox UI/OS and Microsoft is probably aiming to eventually merge the Windows Store and Xbox Store into one.
It's all about convincing publishers to alter their license agreements, nothing that money can't fix, Lol.