If he trades away 2 guys that become above average major leaguers, and keeps 1 that flounders and doesn't, I think that absolutely reflects on his ability to properly evaluate talent, and should be used a piece in the puzzle to evaluate Elias himself. If you want to say that it shouldn't be a major piece of that puzzle, then we could debate just how big of a piece it should be, but your stance seems to absolve him of all blame if he makes the wrong call, and I don't think that's the correct approach either.
I think he should be able to identify who has the best odds. But the best odds don't always hit.
If over the long haul it appears that we're seeing far less MLB performance than we should relative to prospect status, then that would reveal a trend that Elias isn't as good as everyone else at this. Similar evaluation should be done of drafting and international signings. There is a lot of noise in these types of things. You need a big sample to be able to fine the signal.
Expecting that any one decision must be right because Elias is supposed to know these things is unreasonable. That's what you said to begin. Expecting Elias to do a good job overall over many decisions is reasonable.
So a choice between Westburg, Ortiz, and Norby going rightly or wrongly wouldn't in itself reveal anything about Elias. That choice would be part of the overall evaluation, and if he's getting it right more often than his peers than he's a huge asset and if he's getting it wrong more often then he's a liability.