I think part of the reason the JWST took so long to build was the complexity of the design. As in, it was difficult to manufacture and assemble and test due to the way it was designed. It might be cheaper to do it a second time based on the lessons (avoiding mistakes) from the first, but it would probably still be quite expensive.
I wondered the same thing. I once read this commentary on project management (disguised as a StarWars fanfic)[1] which makes me pessimistic. However I'm in no way involved so I'd be curious to hear what someone more knowledgable thinks.