OK, thanks joemikeb. So it looks like it's an Apple design decision.
If it weren't for the fact that come Fall and the release of macOS 10.15 that Sierra won't be getting any more security updates, I'd probably stick with it for a long time.
I'm hoping to acquire the newest 27" iMac available before 10.15 ships so I can stay with Mojave and 32-bit app goodness for awhile longer. I have a few 32-bit apps that aren't likely to make the transition to 64-bit I still want to use, mainly DragThing. My favorite Mac app of all-time, I use it every day, many times a day, and there is no comparable replacement I've found.
