The play: SPD goes populist-nationalist to catch all of the right-wing worker votes and nibble off from AfD/CDU. They leave the open society social-democratic policies to the Greens.
I’m not sure whether in that constellation Green and SPD would be able to work together anymore. In Denmark the labour party has taken a hard right like that and it’s a bit difficult to stomach. Still, I’d prefer those votes to go to classical labour parties instead of going to fascists.