It generally comes down to semantics, nay?

I don't think that the UI is the OS, but part of it. As such, I would say that there are many spatially significant heritage trees of OSs, with degrees of compatibility, structure, interface, similarity, etc, between each node. The further down (or up) you move, the more different the OS becomes for that characteristic to the reference node. Thus, I have found that absolutes don't jive well with OS classification -- which is why it is generally a lively topic of debate. This is the very reason I stick to 'degrees of similarity' in one area or another.

I believe the whole idea of classifying OSs, is like classifying humans -- each case has a multitude of differing characteristics, and thus loses relevance as an activity, unless you're considering a very narrow characteristic.

I remember getting nailed to the wall when I suggested that Android was Linux.... I don't do that anymore