In the case of the PS4 it apparently jad to do with the frequency range of the laser.
Xbox does not use emulation, they did like they did with the 360, the game runs a native binary (from the server) for the target platform... It only uses the assets from the original game.
This is probably why licensing is so complex, they basically do a port, even for those that still own original copies.
Now, if there was an actual emulation layer some games could be put in the store for purchase and download (like Sony already does for PS2), that would be entirely up to the publisher... And maybe, if the game runs natively from disc it could be just used, even if no official support for it exists.
I think that using existing media could work, third parties make old consoles that run NES/SNES even N64... No license.
I think that the approach MS takes has serious benefits, but it also limits their reach.