How do ～以上 and ～あっての work in the same sentence?

Question

彼がやらせてほしいと言う以上、見込みがあってのことだろう。やらせてみたらどうか。責任は私が負う。

"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.

「～以上」＝「～だから、当然・必ず～」 「～あっての」＝「～があって、はじめて可能な～」

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?

Answer 1

「彼{かれ}がやらせてほしいと言{い}う以上{いじょう}、見込{みこ}みがあってのことだろう。やらせてみたらどうか。責任{せきにん}は私{わたし}が負{お}う。」

The first sentence, as well as the whole passage, makes perfect sense. It uses the combination of 「以上」 and 「あっての」 correctly and naturally.

「～以上」＝「～だから、当然・必ず～」

This equation makes me feel unsure if you are understanding the meaning of 「以上」 in the particular context in question. To me, it looks as though this interpretation would be applicable if one were talking about a law or rule making one act in a certain way.

The passage is all about the speaker's enormous amount of trust in 「彼」 and his abilities. Thus, 「以上」 here would simply mean "seeing that" or "as long as".

「見込み」 refers to the (rather high) possibility of success that 「彼」 himself must be seeing in this task/project/challenge. This, of course, is the speaker's conjecture which is clear from the use of 「ことだろう」. Again, this optimistic conjecture is based on the speaker's trust of 「彼」.

In other words, the speaker (who must be 彼's boss) is saying:

"彼 would not be requesting to take on the job if 彼 were not confident enough to pull it off. So, let's let him do it! I'll bear the consequences"

Answer 2

「以上」 from jisho.com :

since ...; seeing that ...​

And the 「見込み」 is the expectation of the one who is asking the be allowed to do it.

So the whole sentence should be :

Since he is saying the we should let him do it, he must have some expectations (in achieving it) (so that he is asking for the permission).