Yes but we can at least grow the city up to the 6th floor. There's so many building that barely reach the 4th.

Japan and Korea have managed to build some of the densest cities in the world on active fault lines. For a first world world power economy like Germany reinforcing prime real estate in their capital in the 21st century is not an infeasible economic obstacle.

I doubt that high buildings help, no matter how many people you square per cubic meter, in big cities it will never be enough to allow affordable living...