[wongarsu]

It's hard to say, but for the JWST the design and R&D seems to have been the easier part [1].

In general it's uncommon to build a replacement satellite after an issue. Either you make a backup from the start, or you start over after failure. Making a one-off component often isn't all that much simpler just because you've already done it once a decade ago (and some parts of the JWST should be about a decade old by now). And if you commit to doing that all over again, might as well update the entire design to new capabilities and requirements.

1: https://space.stackexchange.com/questions/48902/are-there-an...