I really wish NASA would stop doing this prototype-only stuff. If we can justify designing a single sun-shaded telescope operating at a lagrange point a million kilometers from Earth, why not send three and have a bit of redundancy?
Giving this some more thought: I don't think we can justify it. Or if we can, it's only barely. It's definitely in the "nice to have category". If we don't send this up now, we'll send it up later. Or we'll send something else up later. Or someone else will send something up at some point.
And yes, it is possible that
this will be the one that finds the cure for cancer or discovers the benevolent aliens or whatever. Maybe it'll turn out that if we'd gotten off our asses and launched seventeen of them a hundred years ago, it would have prevented the Great Depression, avoided WW2 and the Cold War, and stopped the election of Donald Trump. But I doubt it.
Cosmology isn't really something that calls for redunancy at $1bn+ per item. A large part of the justification for throwing even $10bn at it is NASA's promise that they can get it right (or at least make it work) the first time.
Too, how could you do something like the JWST
without at least one prototype? Even the Apollo hardware designs went through revisions and improvements as they learned the lessons of each prototype they flew.
What'll really happen is that if/when this prototype proves itself, future researchers will consider use it as a basis for future telescopes of similar principles and design. But it doesn't really make sense to fund half a dozen all at once right now. It barely makes sense to use the one.
I hate to say it, but where investing in redundancy makes sense is in national defense. Ironically, we could probably have had redundant HSTs, as that telescope used many components and design features originally developed for reconnaissance satellites. Apparently there's a couple sets of Hubble-grade optics lying around, that could be put into orbit to replace the HST. But... NASA would rather focus on the JWST right now. And I agree with that priority.