I think something like this is needed because `rent control` for existing tenants is already there i.e you are not allowed to increase rent with existing tenant.

In Berlin, there is interesting issue now. A friend lives in a apartment for 12 years, pays only 250 €. He says rent was increased by only € 1 during those years. If he moves to some new place, he will have to pay between 900 - 1000 € for same type of apartment cause rents are raising fast.

So either they should allow to increase rent to existing tenant or control how much landlord can raise for new tenant.

Otherwise, I will be paying 900 € whereas my neighbour next door will only be 250 € for same apartment cause she is living there for years.