I don't think it's a question of being expensive, but more like completely impossible. One good thing is that heating in Germany usually is via radiators that a

I don't think it's a question of being expensive, but more like completely impossible. One good thing is that heating in Germany usually is via radiators that are water fed and the water is heated "centrally". Even in single family homes. Apartment blocks have different options. Some are central (basement heater feeding the radiators) but also some apartments have a gas heater in every apartment individually feeding just that apartment's radiators and gas is distributed throughout the apartment block. If you had to replace every single gas heating system that would be both very expensive and without having any actual data I would gather that you wouldn't be able to retrofit all of Germany with that in time. Wood I would gather is gonna be even worse. While I can see how a single family home would convert (lots of folks have replaced their gar furnace with a wood pellet heater w/ auto-feeders), how would that work for apartment buildings with individual gas heaters in the apartments? These are all concrete buildings where it'd be quite the feat to replace the gas distribution with central water distribution for radiators. And those gas heaters do not take a lot of space and are used even in tiny apartments. Where do you store wood pellets in that situation? Of course there are exceptions where heat is provided remotely already by a "Heizkraftwerk" (e.g. fed from industrial cooling systems or garbage burners).