Stable/static UI is my only requirement
Basically, I got comfortable with the original Gnome (2?) on Ubuntu (up until they forced Unity as the "standard"), and converted my teacher wife to it from Windows 95 about 15 years ago. And although I have dabbled in various distros and desktops/UI's from before that time (classic distro hopper), I kept going back to that thanks to Mint offering that interface with MATE.
My wife would be totally lost now after 10-15 years on Gnome as she has gotten older and less flexible about dabbling in new paradigms. It was hard enough getting her up to speed on cell phones beyond feature flip phones, but Windows Phone 8.1 made it far easier than Android 4 which was current then. In fact I was happy to drop my Android phone after I retired, and did not have keep using biz-approved apps, when I saw how usable WP 8/8.1 was right up until MS yanked that out from under us after a few short years. Then I had to get enough of a handle on new Android versions to ease my wife into that mess, and we both get annoyed by it way too much even now.
If Mint ever drops MATE as an option (or whatever keeps looking/working like it), I will probably have to just not "upgrade" her PC unless it absolutely keeps her from using email on Thunderbird/web browser, and accessing Facebook, and a few other of her favorite web sites.
Ordinary, non-geeky people simply don't have the patience to waste time and effort on constantly changing UI's! And I am just tired of getting bogged down in it much myself anymore when there are other more "eternal" concerns on my mind...