The right thing to do would be to launch a bunch of new ones.
(NASA estimated the marginal cost of launching a shuttle to be $450 million, which is going to be most of the cost of assembling a big telescope. Note that I said "assembling" and not "developing".)
Would it be feasible if they "just" rebuild the Hubble with the original plans? I mean it's 30 years old by now but it's a solid device. That would save on development at least.
DOD transferred two "obsolete" KH-11 spy satellites (which are pretty much evolved Hubbles) to NASA in 2012. They've been in storage since. So the scope for a "new build Hubble" is putting instruments on those sats, launching them, and supporting the ongoing mission. That's still substantial, but much less than a new build from scratch.
The infrastructure no longer exists. The tooling no longer exists. Miniaturization makes old designs obsolete. Design is now the cheapest part of manufacturing.
Rebuilding electronics from original plans which are 30 years old will be very hard. Components will have gone out of production, and minor design changes to accommodate new replacements will lead to a cascade of things which need recertifying.
I don't know if NASA will ever plan to send a mission to repair it since it's already well over it's intended lifetime (by well over I mean several years which is a lot).
Like saying "it just needs to work until Longhorn ships" ;)
In any case, JWST doesn't have UV capability, so it is not a straightforward replacement of what Hubble does. It just able to look at more distant objects because they've all been red shifted into the IR.
JWST and HST are not comparable telescopes. Moreover, the idea that astronomers would stop using one world class telescope just because another one (even a better one) exists is sheer lunacy. Even if a space telescope that was worse than Hubble existed it would still be completely 100% booked in terms of observation time, because there is much more demand for those capabilities than there is supply, by orders of magnitude.
Sure, but the jump is still tempting for the scientific top brass. There's only so much that can be learned in the visible spectrum and only so many Noble prizes, PhDs, tenures and grants to be had. Staying on Hubble is scraping the barrel when the new sexy JWST promises to open whole new fields of scientific inquiry.