Better that the current citizens have reasonable living conditions...
What makes current citizens more deserving than the people who want to live in Berlin but won't be able to due to policies which reduce the supply of housing?
Assuming they have a home in Berlin, the fact that they already live there makes them deserving.
A home is unlike any other material good and the basis of securing a livelihood, it should not be allowed to exploit tenants for profit.
AirBnB'ers and other speculators are of course not deserving at all.
People who own and live in their home in Berlin aren't affected by a rent freeze though. It is only renters, who by definition do not own where they live.
I used "home" in the metaphorical sense in my former comment. A roof over one's head, place where one lives, etc. One of the pillars of our lives, even in times such as these, where people move around much more.
What makes current citizens more deserving than the people who want to live in Berlin but won't be able to due to policies which reduce the supply of housing?