The problem is that the technology that JWST is built with is no longer available. The development cycle moved so fast that the industrial bits needed to manufacture JWST are no longer there, even if we have complete and detailed plans.
Very similar with how building a Saturn V today is logistically impossible, even if we can build more modern, efficient and cheaper rockets.
Very similar with how building a Saturn V today is logistically impossible, even if we can build more modern, efficient and cheaper rockets.