Granada have been planning a move for some time see:
Historic move for Granada TV - News - Manchester Evening News
I assume this idea has either been rejected or is in abeyance.
A move to media city, particularly with the current methods of sourcing and producing programmes between rival TV transmittiing companies where BBC, ITV, Channel 4 and a host of independents produce for each other, seems logical.
The Quay St building, one of the first curtain wall buildings in the North West is not listed and, as the site is in an upcoming and valuable part of the city the chances are that, with current technology, the need for anything large to be built on the site for TV use is very small.
In many ways it will be a shame to see it go but if it has both outlived its usefulness and cannot be modernised for the purpose for which it was intended, there is litlle validity in keeping it, other than as another iconic monument.