Although I too absolutely loved Ambient MATE, the "internal infrastructure" of the "Ambient" family of themes was such that it hamstrung developers who definitely needed to improve the core OS, but couldn't implement those improvements without eventually breaking all the "Ambients".
Again, I too hate change for the sake of change ... but ... change must happen, or the OS/DM will be abandoned by more and more users, rather than being adopted.
To grow UM, the environment must be able to offer advantages that other DMs don't. I believe that having a "safe haven" offering a known, stable quantity (i.e. UM) is attractive in itself for all those who have the wisdom to recognize that innermost need for their primary channel onto the world!
What I think undermines the attraction to UM may be the fact that there is an appearance of conflict between "stable GUI", for what is visible, and "continuity of assured service/compatibility" for the hidden layers of the OS/DM, or their ability to incorporate evolving capabilities that others may wish to add, without supplanting those that exist.
The how for reconciling those two strong needs, and communicate an assurance that they are both delivered with every release, is beyond my experience and skillset. 