In reality, far from stopping the far right, Germany is implementing a far right agenda. Increase police authority? Check. Scapegoat immigrants and other marginalized groups? Check. Build up the military? Check. Suppress protests and dissenters? Check. Impose austerity, providing the fertile ground for fascists? Check.
By the time the AfD comes to power, they won’t have to do anything. The liberals (including socdems and greens) will already have created a fascist society.