Hmm. While this is an excellent description of a downside, I don't know if it's ultimately a bad thing.
You'd figure that in such a situation, the landlords are immensely incentivized to sell, or to lower rents, which may continue to be very unwelcome to them...
...but if the point is that real estate speculation is the problem, wouldn't the looming possibility of that situation result in less speculation, or very careful speculation?
On the other hand, that would hit less wealthy landlords harder (larger percent of their wealth), encouraging them to sell, possibly simply driving more ownership to landlords with deep enough pockets to stomach the (temporary) losses?
Complicated!