> If there are enough vacant units available and enough supply that property values are not appreciating like crazy, someone will crack and cut the rent or sell out so they can get a better return on their money.

Your thesis here is disproven in large cities like the one I live in, Miami. Many investors / property owners here don't give a damn on getting a return on their investment so long as it doesn't go to $0 - they buy real estate sight unseen because it's a good way for the well-to-do in LatAm and Russia to get their money out of developing/regressing economies and into the USA. Half the units in the building I live in are vacant - they are owned by Brazilians, Argentines, Venezuelans, etc who want a safe haven for their money (and for Brazilians, as a base to buy loads of Apple products that they can turn around and sell back in Brazil for a 2-3x markup).