You are right.
My internal SSD is 250GB and nearly always full (arouns 20 to 50 Free GB). Before upgrading, I will baclup that data. So any new mac OS is a non sense excepted the current one.
On the other way, my most recent Xojo license is for 2015r1.
After a bit of thinking, I came to the conclusion that excepted if I have to upgrade (am forced foe some reason to update), I will stay as it is right now. Getting a brand new Apple laptop is a way to “be forced” to use Mojave (probably, and for me).
Nota: Is trongly dislike Apple’s direction (201x) for mac OS.
Beside bugs, I was able for decades to use macOS the way I want and love. Changing Finder grids was boring, changing keyboard shortcuts (in the View Menu) too, changing the way the green button (left of windows) behave, and so on. Not talking about where windows are displayed on multiple screens and too far “features” (changes on old or brand new ones *) are particularly boring.
* The way a senior vice-president make debug (and gives order to change decade old behaviors because he do not know them) - this is IMHO - is particuliarly bad.
All of these reasons (and probably more that I actually forgot) does not let me think I have to upgrade (fast / until I cannot continue to use old mac OS).
Actually, my OS troubles comes with the 10.2 to 10.3 transition (Finder Desktop invisible files), and Xojo was not really good for me (I have to build a simple projedt nearly for every new feature I want to add in main projects until they works there). Sometimes, the project works fine, so I save it, shut down, back home to discover that after I powered on my laptop that the new feature (or another) is no more working. Very boring.