I'm still running El Capitan, I only upgraded from ML to get Maps, I can't see a single reason to run a later OS.
Shame really I've been a happy Apple user since my Powerbook G4, I keep watching Macrumors in the hope they'll release something interesting, but I haven't seen anything in a long time.
Understandable...but a main reason probably many upgrade is due to the programs they use and/or having too more so because of a slick OS...
If you are good with the software you have on Mountain Lion, it is understandable why not to upgrade. There are still good browsers around to use if needing the internet also that work with Mountain Lion still.
But regarding work situations...probably nothing less than High Sierra at this present time (Nov 2020). It is compatible with most recent software, but many cut off compatibility or updates now with Mojave.
Mountain Lion for me also was a good OS and I still use it for some programs that did not update beyond or work better in Mountain Lion (like iDVD).
I actually still use Snow Leopard for some older programs, and sometimes boot up my MacBook 2010 to just play around with Snow Leopard because I just like it and also the overall feel of the MacBook Pro. Both Snow Leopard and the MacBook 2010 is still fast and I just had an overall great experience with it; probably just burned in my memory that SN is good OS. My USB 2 ports are faster than some of my USB 3 port on some of my other newer Macs (including wifi), which I am not sure why...?
I know the census is that we need to update because of security reasons, but if you don't go to a lot of weird or questionable websites and don't download much, generally older OS are secure. We are not at the stage in 2020 with the internet that things "Just" load on your system just by using it...(I am saying generally speaking..i.e. cookies etc.), so I am not concerned mostly with using older OS's to date.