What's wrong with that? Enjoy the little green space you have left while it lasts. Have a BBQ with friends over there, maybe? The gentrifiers/developers won't be so long to remove every last bit of green and freedom in your neighborhood, as they did everywhere else.
...The high housing prices.
It is illegal to keep an apartment empty for more then three months. So if you are aware of any empty housing, feel free to denounce it (https://www.stadtentwicklung.berlin.de/wohnen/zweckentfremdu...). In 2018, 1,9% of the flats in Mitte were subject to an ongoing official procedure on those grounds. The owner can simply claim, they are reforming, but I presume there are limits to that too.
How many flats are really empty is unclear.
I've read that according to the Senat,
- there is an estimated 0.8%-2% of the flats unoccupied
- down from 3.5% in 2011
- ideally, it would be between 2-3%.
The Taz is not particularly strong on keeping opinion from facts separated, but that's where I got it from: (https://taz.de/Spekulativer-Leerstand-in-Berlin/!5749397/)
Two things: I think you put too much value to Friedrichstraße, and likely Mitte in general. It is the physical and historic city centre, but that doesn't have the same meaning as it has in other cities.
Second, in many parts in East-Berlin, you have a unclear ownership of houses and ground with competing claims. People have been disowned by the Nazis, Soviets, or by the GDR. The families have gone into diaspora and are partly spread over the world. Random people may have moved in and layed their own claim.
The ownership is unclear and spread. And it is a battle to lay claim to your _part_ of ownership. The ground is gaining worth over time. All ingredients, which do not expedite the development of land.
From what I understand it is very difficult to do something with that space, either because people refuse to sell or because of bureaucratic red tape.