I guess the Left or the Greens or the useless SPD could theoretically win the elections.

It won't happen though. It'll be AfD.

There was already a left party in power. It’s called DDR (East Germany). We all know how it ended. Any government with an extremist party is not a good government, regardless it being left or right.

But then again in a democracy the politicians must listen to peoples vote instead of preaching them and if that has happened in Germany, they they would already be having a center right government instead of the current circus