The calculation is not made with the governing parties and SPD and Greens only. There are the Left Party and your Pirates, too. Before the ascendancy of the Pirates, polls showed a safe Red-Green majority. The Pirates took that away, and then (thanks to Merkel's Euro grandstanding and a now amost completely uncriticial media all the way from Süddeutsche to the right I'm afraid) the CDU started to pick up. In the latest poll, from Forsa, CDU gets 38%, SPD 26%, Greens 14%, Left Party 8%, Pirates 7%, FDP would fail at 3% – so parties uniting about 46-47% of the vote between them will have a governing majority. The problem is that neither the SPD nor the Greens show willingess to consider either the Left Party or the Pirates. That strategic stupidity and cowardice in the face of potential red-socks campaigns leaves Merkel with all the options.

(Personally, I am thinking that Red-Green with both Left Party and Pirates entering parliament as opposition would be the best option: it wouldn't force the Left Party and the Pirates into foul compromises as coalition members, while as opposition parties they could draw the Overton Window left.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}