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.
Cost is an issue -- you said so yourself above, and you've said it again in this post.