AFAIK the main problem is that Unix's file permissions do not cover Windows' permission model. That would be tolerable on a data partition, but a system partition is going to use all kinds of very particular permission setups on system binaries etc.
You might be able to model that stuff as xattr, but then it could be problematic to mount that ext4 partition into Linux because applications might be copying files without respecting the xattrs.
>AFAIK the main problem is that Unix's file permissions do not cover Windows' permission model.
Well, since Microsoft has been borrowing more and more ideas from the Linux ecosystem, it would not surprise me that a Windows 10 successor would include some kind of compatibility layers for different file systems.
Why don't they just replace Windows with their own Linux distro? :D WSL2 cannibalizes Windows from the inside out, and all that's left is Sphere. Seems like the most efficient solution.
You might be able to model that stuff as xattr, but then it could be problematic to mount that ext4 partition into Linux because applications might be copying files without respecting the xattrs.