shoot my idea down please

I have an idea regarding the continued use of the Space Shuttle and I need a reality check and hope ya"ll will provide.
My idea is to have the Space Shuttle remain in orbit after completing its "last mission".
The way it could be done is that it would have to carry up a Russian Soyuz or progress capsule in the cargo bay which is how the crew would return to Earth. I think it could be done in conjunction with a regular crew delivery mission to ISS. Getting it in higher orbit is the tricky part. With in-orbit modification it could be done with PAMs (Payload Assist Modules) that send satellites from the Shuttle to Geo-Synchronous orbit.
I think it could serve in at least one of several different ways. It could be offered as a kind of X-Prize for any private venture/quasi-private space endeavor ie... a space station like destination for rich tourists. It could serve as a kind of last resort rescue craft for ISS if it is parked in a higher orbit. With in-orbit modification it could become a vehicle for getting to the asteroid belt or Martian moons. It could serve as an actual space tractor trailer retrieving material or refined product from Japanese Lunar mines/factories and delivering them to the ISS or another Space station that might be able to use the material for whatever purpose.
It could become a target for practicing/testing techniques for emergency or impromptu de/construction. Lastly if nothing else it could be sold in-orbit to another space faring country or organization like ESA.
Thank you for your attention