A replacement might actually easier to build. The detector is no small feat, but the major problems with JWST come from the foldable heat shield and the segmented mirror. Both of which are due to size constraints during launch. Basically it has to fit a 20 x 14 m heat shield and a 6.5 m mirror into the 6.4 by 4.6 m fairing of an Arianne 5.
Targeting a rocket with a larger fairing diameter (even at same payload mass) would make the design much simpler. An upper stage with 20 m length and 9 m diameter on a Falcon Super Heavy would allow to have a fixed mirror and heat shield that is only folded once, instead of the crazy origami that JWST is. But of course such an upper stage doesn't exist yet. SpaceX was only started 6 years after the work on JWST started...
Targeting a rocket with a larger fairing diameter (even at same payload mass) would make the design much simpler. An upper stage with 20 m length and 9 m diameter on a Falcon Super Heavy would allow to have a fixed mirror and heat shield that is only folded once, instead of the crazy origami that JWST is. But of course such an upper stage doesn't exist yet. SpaceX was only started 6 years after the work on JWST started...