There was money to be made in housing 10 years ago when rents were half of what they are today. What changed is the demand / supply ratio. I guess the laissez-faire logic would be that higher demand drives up the prices so much that investors can't help themselves but build more. Do we have empirical data from other cities that show that relationship? I.e. as rental prices rise so does private investment in housing? And also the opposite, how much do rent caps like the one in Berlin influence housing investment longer-term? I'd argue our Berlinian experiment did not get a real chance to run, given the expectations around it were that the courts might over-turn it, as they did.
From my understanding and from I read over on freakonomics a lot of the assumptions about rent caps come from single neighborhoods in NY which I'd argue does not generalize.