This is where close partnering helps. Microsoft can lead the way in building all their games(a considerable library, especially after the Activision acquisition) with universal binaries, and provide great tools to third parties to build games the same way.
The games that aren't recompiled will (after 2 years or so) fall far enough behind in requirements compared to the state of the art, that the emulation will be fast enough to play those older games.
I doubt Starfield will ever be emulatable on current gen hardware.
HOWEVER MS does have an ace -- they can make the next xbox an ARM machine. This will guarantee that all game devs either migrate to ARM or face being on an emulator with reduced capabilities. That will create the needed tech. But it will also help Apple... so MS is likely to try to avoid it.
Starfield should be barely playable if someone manages to get it to run(game requires x86 instructions rosetta doesn't currently support). With M3 around the corner it should be very playable, which falls in line with the 2 year timeline.
The games that aren't recompiled will (after 2 years or so) fall far enough behind in requirements compared to the state of the art, that the emulation will be fast enough to play those older games.