...someone correct me if I am wrong - when one "upgrades" from ANY earlier version, the only option through Apple is its latest OS (which, currently, is High Sierra).
That's correct if you go via the standard App Store route, but Apple also offers earlier versions via, for instance, the link I posted for users who don't want to go all the way.
Granted that they make being a laggard difficult, but they don't make it impossible.
The only way I know of to go back to an earlier version is if one has access to a copy of that previous version (e.g. El Capitan) which was made on a separate bootable drive prior to installing said version.
There's no need to make a bootable drive; all you've got to do is archive Install macOS Whatever.app.
There are several websites out there offering download versions of older Apple OSs. But, having never tried, I cannot vouch for either their reliability or legality.
