True enough, Hubble is doing some great science.
But it's also a satellite, and satellites get old and eventually die. Whatever else it may be, Hubble is also an aging spacecraft. It's got a bunch of holes in it from debris and micrometeoroids, and its MLI blankets are old and discolored (which means it's warmer than it used to be). Its electronics have been bombarded with radiation for close to 15 years, which takes a toll even on shielded components. It's been through a bunch of thermal cycles, which causes stress and that affects things like connections and structure.
You are going to have a long wait if you are waiting for a US moon based telescope. Right now we have 500+ billion and growing budget deficets.
You have to be careful when you start tossing costs around like that. What you're really proposing is further Shuttle missions to repair Hubble. Well and good, but bear in mind that it cost over $300 million just to launch a Shuttle even before Columbia burned up -- it'll cost more now. The replacement hardware might cost more than the launch itself (as has been the case in the past).
At some point, it becomes cheaper to build and launch a new telescope, than to maintain the old one. NASA's cost numbers suggest that the threshold has already been reached.
Space is a dificult envirnoment, Hubble appears to be handling the enviromnet very well and thier is not reason to believe if serviced, Hublle can't keep doing great science for many many more years.
You and not going to get another great American Space telescope for a long long time. The country is broke, there is no money to do things like go to the moon or building new shuttles. If we lose Hubble, we will not be able to afford to replace it. The counrty is at war and broke, going deep into debt, the dollar is falling, and entitlements and interest on the national debt consume almost all federal dollars. Don't be fooled by the going to the moon scam, ain't gonna happen, we can't afford it.