Wouldn't the state have to pay a lot of money it doesn't have back to the Feds if it doesn't start work on the extension to the Alexanderplatz by next year?

Agree, it would make much more sense to continue building on the other end, and connect the Hbf/U55 to the heavily-used U9 at Turmstrasse (where the 'U5' station is partly built already). And I'm sure that they will do something with the area TXL occupies once the airport closes that may merit a subway connection...