Why Mate instead of a modern, extendable DE such as GNOME 3?
Have you ever pushed a major application re-design out to 'normal' users?
Disclaimer: personally, I quite like Gnome Shell running on top of the Gnome 3 libraries and gdm3.
It's funny you almost never hear about companies having to train staff to use a company smartphone. I guess all smart phone interfaces are pretty similar though.
Mod4+Type and get the browser working
Then the names of the programs (and how to find applications in the activity view). Many people navigate by position in menu.
Then the change in Alt-Tab/Alt-` behaviour (could be fun) and the new workspaces (not many use those)
Then just being around when odd combinations of old workflow and new one happen. Minimising a window and forgetting the application is running. Nothing to click on to get windows back &c
I'm not defending anything here, just answering the original question.
Also, the assertion that GNOME 3 is extendable is funny. It's probably the least so out of the major GNU/Linux DEs, as the developers have been hellbent on pursuing "brand coherency" as of late, and in general dissuading from tampering with defaults.