Berlin pretty much built out? The city has a huge "hole" in the center - Tempelhofer Feld. 355 hectares of land that at some point it was decided to leave untouched. Complete waste of space that could be used to build thousands of units.
The idea that the Tempelhofer Feld would have become anything but a mall with adjacent luxury apartments is bonkers.
The German constitutional court had just forced Berlin to sell of most of its housing infrastructure and the mayor and his confidants clearly were either unwilling or incapable of enforcing laws that would have ensured some form of public control and affordable housing options.
This was the time when people were offering tours putting all these vacant buildings on display that were left there waiting for the rent moratorium to end. When corporations freely redesignated large and central areas to the next biggest mall and luxury apartment project.
Edit: wording
Tempelhofer Feld is needed as greenfield in the central city for climate reasons.
Ideally it's a compromise, everything gives a little. A few more areas for housing, a little higher buildings, a bit higher prices, a bit more attractive rural areas.
Manhatten is one of the densest populated urban centers in the world. Nearly no other green areas, even street trees are a rare sight. Everything is concrete. Berlin on the other hand is very green. Parks, trees on the street, Tiergarten, Grunewald, and yes, Tempelhofer Feld. You could build housing for tens of thousands of people while still keeping the area green with lots of trees and parks for people to relax in.
Just keeping it a huge empty space and at the same time complain about high rents is ridiculous. This is not solved by complaining about money hungry investors. It's the supply-vs-demand ratio that's the problem. Not greed. (It may play a role, but would not have much of a chance without the underlying issue.) Of course you can keep it "for climate reasons". But that comes with a price and you need to live with those consequences. "Mietpreisbremse" is not going to solve any of those.