It is because even they come via XAML Islands, or UWP/COM interop, at the end of the day it is still UWP/COM that is going forward.

Looking forward how Windows will look like now that it was officially communicated that Windows 10X is also coming to desktops and laptps, with its sandbox model for 100% of all userspace.

All that is irrelevant, the point is that having a frozen API doesn't mean that the API's implementation is also frozen. You can still get new features on a frozen API as well as new APIs alongside it.