“...when the Romans quit, they did so because of diminishing returns (IOW, there was nothing worth the cost of conquest, or, there were much richer pickings elsewhere all of a sudden).”
How about this for a limiting factor - they stopped when they didn’t find any more natural warm mineral springs where they could indulge in the Roman bath experience? This observation could apply to Germany and England. I suppose if they ever heard of Iceland’s warm springs, they would have launched boats and tried to run off the Vikings and their descendants.
You don’t need warm springs to have warm water. The Romans certainly knew how to heat water.