I live in a city where they buy existing flats and houses and rent the to citizens. City is also buying building ground to tie the sale to investors to conditions like the amount of social housing in the new building area.
Works like a charm. The rents are okay (even from other landlords since they have to compete) and we don't have "problem areas" in the newer parts of the city since people live in mixed areas.

Fingers crossed Berlin works out. It would be a welcome change for a lot of friends of mine to finally have an affordable flat that doesn't rot away.