r6, I use mountain lion because I prefer the Steve Jobs era look and functionality. Correct me if I'm wrong, but you can't easily skin say Mojave to look like mountain lion? Also, I actually use dashboard, notepad, I use gmail with apple mail, I use the old itunes. I sadly think Tim Cook is destroying OSX, for real.
And the funny thing is, I do the converse of what you advised. I use a more recent OSX and windows 10 virtual machine running in the background to run stuff Mountain Lion won't, including Google Backup and Sync. The hacking that involved, loads of fun! Also puts my 48 gb of ram to use. I also found a software to remote desktop to and from other computers running modern OS's. (I'll spill the beans for anyone interested: Splashtop)
Anyway, cheers!