Maybe someone can help me with the subtle details, but it seems to me that instead of just replacing Hubble with a upgraded equivalent why not bring in Europe and Asia (Japan is already onboard) and produce a truly next generation optical/UV design.

I'm assuming that the largest launchable monolithic mirror would be in the 4m range - based on Cassini's fixed 4m main antenna. By my calc the image integration times would be 2.78 times faster (at much better resolution) - I think its safe to say that even with extra partners U.S. observers will not have to reduce their proposal allotment from current levels. Scaling the mirror diameter will exponentially increase its cost - the cost for the 2.4m HOP optics are in the 200M USD range, so a 4m set of optics will be around $550M. If the rest of the program cost scales linearly to about $1.3B the total program cost will come in at under $2B.

If the Europeans and Asia come in at 25% each the U.S. will get a much better instrument for the same investment - not to mention the social benefits of international cooperation in the pursuit of scientific discovery.