IIRC Philip planned to ship her off to Savoy not Austria.
Also, if Mary lives until 1577 Philip will desperately need a heir, assuming Don Carlos still falls.
This might also prolong the last Italian War; since England was brought into the war by the marriage to Philip, Calais was a legitimate target for the French, and they took it. OTL, she died before the end of the war, ending Philip's claim to Calais, so he had no problem signing it away at the peace. If she lives longer, the war will probably continue until it can be recaptured.
Calais was too much trouble to be worth of being kept by England.
Calais was too much trouble to be worth of being kept by England.