SO… I might have found something at least where in my current game.
The desirability seems to be directly linked to repairs/condemns.
When I made the first post in this thread, I was also struggling with abandon houses. But my Pubs and Healers were condemned so it was hard to get up. I cleared the Pubs and Healers, rebuilt, and added some decor and the abandons filled back up
AND THEN… All the yellow repair needed status bars, and the red ones with condemned buildings started going away. not all at once, but I could see them working on them and then they would go back to 100%. These building had been degrading or condemned for YEARS, and now in the matter of a few months, almost everything is cleared up. So I don’t know if this is actually a bug, or the way it was intended.
The only repairs I am worried about are things like my manufacturing and barns that are in areas of very low desirability. Will update in a little while on that.