I'm not behind the rationale for this proposed change. Very much not behind it.
It might be time to take another look at what "Ubuntu-MATE" represents, as a reminder if nothing else. There are two parts to it, the Ubuntu part and the MATE part. The MATE part is mostly about the desktop. And the desktop is mostly about paradigm.
The reason for MATE was the mainstream's (ie, Canonical's) adoption of Unity, right? The mainstream went in a direction some folks didn't like, and some folks decided to build MATE as an extension of the gnome-2 panel manager. Or am i mistaken in the whys and wherefores? I could be entirely wrong about that.
Anyway, at the present time, that same mainstream is going in another direction that maybe some folks don't like. Canonical recently announced some changes in Ubuntu.
I guess what i'm saying is, where's the commitment? How tied is Ubuntu-MATE to what the Canonical decides to do with Ubuntu? It took a while when Unity was shoved in our faces for the community to get sufficiently fed up to come up with MATE. Are we able to learn from that experience, and relate it to the new direction Ubuntu is taking, or are we going to experience the same thing after a while?
Aside from that, some people know how computers work, and some people know how to work computers. One of the things i like about the MATE paradigm is that it assumes i know how computers work and lets me do work. One of the things i dislike about other paradigms (like Unity) is that it assumes you know little or nothing, and makes you figure out how to work it before you get half a chance to do any work with it.
Now, you said the rational was to create a compelling OOTB experience. I've been writing code since the winter of 1969 and for me, the most compelling experience possible OOTB is for the OS to isntall gracefully on whatever hardware is available, and display something that speeds me on my way to getting work done by editing files and running programs.
You also said the rationale would allow UM to more effectively "compete with the other Ubuntu flavours". I'm absolutely against competing and other forms of one-upmanship. Look at what competing has done to Microsoft and Apple in the past, their proprietary competition has made each of them a group of former users that despises them, and guess where that group is migrating to.
I've always had the understanding that linux and FOSS in general was not about becoming better than the (mostly-lame) competition, by inventing a wigglier paper-clip (showing you can play their game better diverts you from your own game), but about producing the most perfect computing tools possible.
JMO, apologies for anything that might sound negative and thus unfashionable. I'm of the opinion that once you know who you are, you know what to do. Thus when what to do becomes an issue, looking at who you are might be useful.