Re: HST lifespan
It's our heritage. We owe it to our descendants to put it up in the Smithsonian.
On a practical side, I don't know if TDRSS could handle (technically or politically) the traffic of two civilian space telescopes plus god knows what else..... It'll be old, tired, and technically very obsolete. NGST will be able to pick up the slack I'm sure.
In fact, the more I think about your question, the more disturbed it makes me. If Belgium wants to have a sale on diamonds and chocolate and put up a space telescope just to jet it burn up in a kinda pretty mir/comptonGRO light show, be my guest.