To be honest, I don't know about the payoff. But I do take some issues about the direction NASA is taking(or being pushed by politicians). The thing is that the longer they stop doing space mission, the more difficult it will be to take it up again. People die, they forget, they quit. And then suddenly after 15 years NASA have to invent the wheel again when they want to take up manned space missions again. The most valuable thing NASA have is the experience of their employees. the fact that they landed on the moon 40 years ago is probably almost useless today. It would have made more sense economically(which should not be the only consideration) to continue they moon mission to present day. We need to build up experience with manned space exploration. It doesn't just magically appear by sending up probes.
I agree with you that a lot of bad decisions have been made, both by and for NASA. However, it's too late to undo many of those decisions. Whether we like it or not, the US has lost the manned space flight ability for some years to come. Pursuing a failed project now, just because we lack a non-failed project, is just throwing good money after bad.