I appreciate that THF is a historic landmark (and, along with the Olympic Stadium, one of the only remaining examples of large-scale Nazi triumphalist architecture) but is not the real issue that they'll be closing Tegel at some point? Only slightly further from Mitte and the governmental district than THF, it's an ideal city centre airport with the added advantage of having full European and transatlantic capability.
It'll be a pain to have to schlepp out to BBI/SXF in future, high-speed link or not.