If the Hubble isn't upgraded, then it will be caught up by other telescopes. There are many under constructions and others for space too. Once it's caught up, it will loose its appeal.
The key thing is (well, I read this somewhere) in astronomy, if an instrument isn't somehow upgraded within 3 years, another telescope will produce beter results.
The HST has the advantage of being in space. However, it's so close to Earth it forces its gyroscopes to work enormously just to keep track of a specific point in space. And its lifetime depends on the gyroscopes.
So, further away from Earth (like L2 or even Jupiter's L4 or L5 orbit) is a better solution.
But it's true the HST is the most successful Shuttle-related science mission.
I think what matters most are the discoveries, the science and not the telescope which is a mean.