I had to split this to its own topic as it moved away from the "top 3" wishes for 20.04.
I think if you ran a poll, I bet an overwhelming majority certainly would not want Ubuntu MATE to become locked into a certain workflow. 
Customisation is a really good strength of MATE/GNOME 2.
I agree with some of the points that it might help optimise resources by focusing on fewer areas for error, but really, MATE works well today for a lot of user's ideal workflow. Something arguably not the case when working between mainstream OSes. Everyone can be unique here! 
Trouble is, MATE is more of a desktop environment then a desktop experience.
Windows, Mac, GNOME and KDE have their own login screen, window manager, GUI toolkits that allow them to properly define their vision of a desktop experience. Arguably they could be heavily customised with extensions and mods. MATE instead fits like a jigsaw to a distro maintainer's combination of components.
The question could become... should Ubuntu MATE just trash its default options and be a plain implementation of the MATE desktop? .... Nah. That's not the objective of Ubuntu MATE.