I think the problem is expectations. Just maintaining a distro is hard work.
The MATE team did not only fork Gnome 2. They cleaned up the code and added features.
Then the long hard road to a GTK3 version of MATE began. As I understand it this has been finished after more than 2 years of hard work.
Well, since GTK 3 is a rolling release, just maintaining a GTK 3 version will require considerable work, updating themes etc.
Then releasing a new version of Ubuntu MATE twice a year. I don't envy those who work on (Ubuntu) MATE. But I am still happy they do it.
Ubuntu 9.04 was actually my favourite Ubuntu release ever. Even if there isn't that much difference between Ubuntu 9.04 with Compiz/Emerald and Ubuntu MATE 14.04 with Compiz/Emerald on the surface, I know that a ton of work was needed to give me the same experience as Ubuntu 9.04, five years later.
Actually UM 14.04 is better than Ubuntu 9.04. After 14.04 many things happened; systemd happened, GTK3 happened, Intel BayTrail bug happened, Network problems happened. Except for GTK3 most of these things are beyond (Ubuntu) MATE control.
I think it's all about expectations. I don't expect the next version to be better than the last. I expect it to be different. With so much under the hood engineering taking place in Ubuntu since 9.04, 14.04 was kind of a miracle to me.
I thought desktop Linux was lost when Pulseaudio was introduced in Ubuntu 8.10. Pulseaudio could easily be uninstalled in 9.04 (maybe even later).
My point is there has been so much change since I started using Linux, that I don't really expect anything. I don't even expect Ubuntu to deliver 18.04 LTS (there are no more letters in the alphabet) . If they do that's great. If they don't, many distros will just rebase on something else. Linux in a nutshell, I think.