It costs many millions of dollars each year to operate and maintain the Hubble. This is what, the 4th (?) dedicated Shuttle flight for the Hubble counting its launch in 1990. Each of these Shuttle flights costs several hundred million dollars. There's also the operations staff and facilities and other continuing costs for Hubble. Add to that TDRS satellite time, facilities and equipment maintenance, etc. and you're talking about a substancial amount of money.

My guess is that Hubble will probably be decommissioned not too long after NGST goes online and completes its checkout phase. I doubt there will be enough Operations & Maintenance (O&M) funding available to support both systems. Given the choice between letting Hubble burn in or bringing it back for the NASM, well, NASM is a better choice IMO.