The HST was conceived for the Shuttle.
It was meant to be put in the highest orbit possible for the Shuttle, knowing the mass of the HST and at its lowest possible inclination, from Cape Canaveral.
The HST was especially built to have parts of it removed and replaced with better instruments.
There is no other vehicle than the Shuttle to improve on the HST. If one were invented or proposed, it would take much too long and the HST simply can't wait too long, because of the gyros; or the other vehicle couldn't take the replacing instruments with it or couldn't get to such an orbit.

Besides, it's not because the ISS can be used as a safe haven, in case of problem with the shuttle, that it will always be available. ISS bound Shuttle flights could also fail and be unable to reach the ISS or return back to it a the end of a flight.

Raoul