I think MS in the middle of an Xbox renaissance with new focus on first party, game pass, play anywhere, etc. I don't think what we're seeing is them gearing up for next-gen, rather them still grinding out changes and games behind the scenes that we don't know about yet.
I believe there's going to be a blur between current and next gen so you're all right. Most of their newer projects will be developed with leveraging the next Xbox in mind. Despite that, there's no reason they can't scale down to work on OG Xbox One and Xbox X. Games like Tomb Taider, Titanfall, Destiny, Forza Horizon 2 and MGS5 had 360 versions. It takes a few years before a new generation's games get to a point technically where they won't scale to previous generations. I'm not sure you can find more than 10 games this gen that couldn't scale down to the 360.
In addition, one of the points of UWP is scalability. Even if next gen has CPUs orders of magnitude more capable than OG Xbox One, games should scale from next gen machines down to running on a potato. Microsoft wants their games to eventually run on tablets, low powered PCs or anything with Windows 10. That's part of their not so secret plan with Game Pass.
So Microsoft is trying to attack multiple objectives at once. They'll be leveraging cloud compute gradually. Building games to scale across platforms. Rebuilding their portfolio. It all ties to their end game which is being able to sell their services to users over multiple platforms. Higher stakes due to far more potential reward so it's a fair assumption they'll be extremely aggressive.