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.

One of them is, as far as I know, now dedicated to WFIRST:

https://spaceflightnow.com/2016/02/18/nasa-moves-forward-wit...

It was just the mirrors. Earth observing telescopes aren't useful for deep space.

The infrastructure no longer exists. The tooling no longer exists. Miniaturization makes old designs obsolete. Design is now the cheapest part of manufacturing.

(See "Can we rebuild Saturn V in 2018") https://www.youtube.com/watch?v=mhIfeS3OumY

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.

It took over 2 years just to make the wave mirror.