O'Keefe Under Fire for Hubble, ISS Decisions
the new telescope cannot be fixed
Which makes it just like every other scientific satellite and probe with the exception of HST.
It's proven cheaper to build a completely new satellite and launch it than it is to have a repairable satellite. Repairing means having to build the satellite in such a way that an astronaut can get to the components, and change them, which means:
1) Larger. A component which is designed to be replaced has to be accessable, which means you can't pack them together so well. Everyone knows it's harder to repair & upgrade laptops than desktops, because the laptops are built to be as small as possible.
2) Heavier. Same reason.
3) In a possibly less useful orbit. JWST couldn't do what it's going to do in HST's orbit. It would be too warm, and too prone to tidal variations.
4) Sometimes not repairable after all. HST's mirror was never replaced, it's got the original, misbuilt one. It's simply too big & difficult to replace the mirror.
The cost of getting astronauts up there is so huge, that added to the expense of 1 & 2, that means that it would actually be cheaper to have launched a series of HST's, than to repair & upgrade the HST.