Meh. I have friends who lived in a Vonovia apartment in a prefab in East Berlin. It's not for me, but the rent was pretty cheap (~10 Euros/sqm, heating + warm water included) and didn't get raised during the years they lived there.
Berlin just came from a place of super low rent and is slowly catching up to other large German cities. Understandably, Berliners are in shock, but it's more "things are returning to normal" than "Berlin is so expensive, nobody can live there".
The simple fact is that the people who complain about their homes they rent from those companies are unable to find other accommodation for a comparable rent. Which in turn implies that it is not really the fault of those companies, otherwise somebody else would be able to provide better housing for the same price.
Berlin just came from a place of super low rent and is slowly catching up to other large German cities. Understandably, Berliners are in shock, but it's more "things are returning to normal" than "Berlin is so expensive, nobody can live there".