Over the years, I have had many problems with interoperability with mac hardware and OS versions.apple periodically releases, for example, security updates for older OSes.
also, since you're still missing the point: we get incremental updates that happen over the year to address bugs; it's not like apple releases an OS on a tuesday, then heads to barbados with a bag of weed and the harry potter books for a year, while we wrestle with OS issues...
Right now, I can't hot-disconnect my eGPU from mini2018 & 10.14.6. I'll have to do full shut-down, then power off eGPU and then restart the mini. This is fixed in later OSses, but not in Mojave.
This is what they do: they don't fix all the bugs in older OSses. Only some of them get fixed in "supplemental updates". Also, fw updates and security updates breaks new thing in old OSses, that are not all fixed. All this would change to better, if there would be longer period between major releases.