too vague for me
Why not a bridge to the ocean bottom or to the deep underground - we can wave our hands about how those things will also pay off someday.
I don't think it will happen, but the closest thing to a politically-viable way to kill off Nasa's manned program would be to transfer the money to materials science in the states currently getting Nasa's money, mostly Texas and Florida. That would build much better bridges IMO.
As for manned space exploration not happening, we had a little private launch last summer. It will happen, the question is how quickly. Nasa's way isn't worth it.