Just an idea but if Lightroom (not Classic) supported file short cuts / aliases we could build our own file system based collections without having to duplicate the actual image and xmp files?
I'm not an Adobe developer, just a Lightroom user like you. But I have no idea what you are asking for, more importantly I don't know what problem you are trying to solve with your suggestion. If you could explain the problem and your solution in much more detail, it would be very helpful.
I've just realised that you might be referring to the new Local Mode in Lightroom, is that correct? In which case I think I do see what you are trying to achieve. Whether Adobe would want to extend the Local Mode in that way is another matter, but I guess that's what the "Ideas" section of the forum is for. It'll be interesting to see what sort of traction your suggestion gets.
Hopefully since Local Mode has been introduced there will be further development with the introduction of new features.
I've also found that if a file is passed to Lightroom from Photo Mechanic for example then there is no option to process it locally, it needs cloud sync.