"There must be something to it if he's saying that we should let him do it. How about we just let him? I'll take full responsibility." At least that's how I'd understand the sentence. What I'm unsure though is how ～以上 and ～あっての work together.
I just don't see how these two are supposed to work together here. "Because he's saying that we should let him do, there must be something to it - but because there is something to it (expectations for the future?), he's saying that we should let him do it?" You see what I'm trying to get at here?
Also how exactly does「見込み」work here? Is it the expectation the guy, who wants to be allowed to do it, has or is it from the point of view of the speaker of the sentence?