Arguments:
The HST is 33% of what NASA shows to the public, it inspires and it's pure science.

A Shuttle to the HST won'r dock to the ISS, but if there ever was a big problem to a Shuttle docked to the ISS, can the station sustain 8 or 10 astronauts for 2 or more months, in space?
Any potential problem to the Shuttle will ground those left on the Earth, if only to fix it while it can. This will mean a long time will pass before a shuttle with only 3 astonauts could go up to return 11 of them...
The ISS as a safe haven isn't workable, presently.

If the Shuttle is fixed and the 3 next flights have no problem whatsoever (yet I remember STS27 with lots of tiles damaged), if the HST is abandoned, it's not going to sound good: here we have a safe shuttle but we can't repair the HST anymore...

If a robot does indeed save the HST and enables it to perform a lot of great science for more years, then the human presence in space becomes even less defensible, at least for government projects.

However, if the HST can't be ugraded, it will quickly be surpassed by ever better earth telescopes. It might die not because of a technical problem (on the ground or in itself) but because of it being forgotten.

Raoul