The old code didn't work after the port to GSettings. It crashed in weird places. That's why it was "#if 0"'d around in the GSettings port commit. As I said, the original intention was to fix it, but minds were changed.
With hindsight, yes, we'd love to do things differently. Saying "you should have done" doesn't really help the current situation here.
The gsettings port broke stuff and reintroduced problems. It was long overdue. This means that cutting features to get things out of the door might happen.
If I had to guess without debugging, I might guess that the 'changed' signals were firing at different times in comparison to when the slider was being adjusted that means that changing the background wasn't ordered 100% correct. The code stems back to 1996 -- it's possible that assumptions like these are being made.
So what does it take to make the minds change back again?
When I stare at opaque terminals all day I get severely perceptually understimulated to the point that I fear becoming an automaton.
But with some sliding animations and a bit of flower poking through it soothes my nerves.
Is that not enough reason?