Constellation is about going into deep space again. Starting at the Moon, go to Mars, and beyond in the late second half of this century likely. The Shuttle is not a part of those deep space purposes. Building a potential future platform for deep space missions in low earth orbit will be done way cheaper by another launch vehicles rather than using the Shuttle for it. NASA these days realizes that assembling the ISS using the Shuttle was technlogical milestone, but it was also somehow s****d to put it up in such small parts using something like the Shuttle. I don't remember who but a NASA responsible once was quoted "I hope we won't be that s****d again".
The Shuttle is out of the business in any case if the future is about going beyond LEO. Let's enjoy 7 more missions and call it a day. Any program has its end, and to be honest, the Shuttle lasted for too long anyway. Constellation should have taken place more than one decade earlier already. But now NASA is in a real mess, which includes the follow on program forced by the STS-107 accident, while there are pacts to build and operate that big and fat space station. Both, ISS and Shuttle have to go if NASA seriously wants to do something totally different, i.e. going beyond LEO again.
