Quote Originally Posted by chscag View Post
But as Jake said, you really don't want to fall too far behind with macOS because sooner or later you'll be scrambling around trying to find something that works.
It's true that you don't want to fall too far behind. But unless a mission critical app makes a major change in file format (rare), or disappears altogether (sometimes happens), or there is a brand-new app that appears that you are very interested in that isn't backwards compatible (sometimes happens), you can probably go for two or three versions of the Mac OS without upgrading and be fine.

Have a look at:

Upgrading To The Very Latest Macintosh Operating System
Upgrading To The Very Latest Macintosh Operating System