What would another James Webb Telescope would have cost? Far far less then the first. Sure it is costly to destroy a prototype, but not as much as you think.
That only works when you're doing something like Starlink or Starship's development, where losing some prototypes is fine. However, that's not how JWST was designed, so losing many prototypes of it would be very costly. If JWST were designed with the consideration that it'd be mass produced, it would not be the JWST we know today and instead would be a completely different design (a rough comparison can be between OneWeb's more standard delicate satellite design and deployment procedure vs SpaceX's heavily optimized stack of less delicate satellites just drifted into orbit).
Most space companies are still doing their prototyping in the JWST style, so losing those prototypes is costly.
It was a huge publicity boost for NASA, but not actually an efficient use of resources in terms of % shuttle missions * total cost of the shuttle program. That’s arguably unfair because of the sunk cost issue and how the budget was setup, but manned missions are inherently expensive and even more so if you’re trying to match arbitrary orbits.
It would have cost less sure, but it would have delayed the program years. And such a delay could easily shutter the program if political support faltered.
The companies launch prototypes because they want to test out technology and build methods that will go into the production satellite. Hence losing the prototype is a huge loss in time and/or increase in risk (if they decide to go ahead with certain technologies despite lacking on-orbit validation).
Most space companies are still doing their prototyping in the JWST style, so losing those prototypes is costly.