Trades can be evaluated two ways, and they are different.
1. The transaction itself - was it a good decision? Was the rationale and balance of immediate benefit/pain vs. future potential benefit/pain at least clear and arguable?
2. The outcome - what ended up happening? In retrospect, did the trade work out well for one side or the other?
Those two things can be and should be separated.
Even in separating them, which makes the discussion somewhat clearer, we are dealing with the future, which is of course unknowable, and we are dealing with counter-factuals (alternate timelines) which are all speculative at best. Only in rare occasions are trades 100% clearly "won or lost" using the outcome to evaluate them. However, in the immediate transaction it's a bit clearer whether the rationale is sound.
RE: the draft pick discussion, based on a reasonable market value of draft picks (the outcome of the players drafted at that position, a team's development ability, etc), you can evaluate the transaction.
You can evaluate the outcome (how that specific prospect turns out) much later on, but it doesn't change the evaluation of the transaction itself. You could have a great transaction and then the draft pick fails and it was still a great transaction. Or, you can have a poor transaction but you get lucky and the draft pick hits, and the outcome turned out great. Doesn't make it any better of a transaction, just got lucky.