> People with a rent controlled unit will not leave even when they should, because they have it so good.
... What do you mean "when they should"? If they don't want to leave because the rent is OK and they like living there, why should they move?
I guess one fundamental difference with Spain VS US or probably many other countries, is that here in Spain we have "the right to housing" enshrined in our constitution. This makes it so it's more important for residents and citizens to be able to find housing, than it is for owners to be able to make profits on owning properties/land.
The laws in the country should reflect this, and thankfully, they're starting to, albeit slowly.