I think that when trading for prospects, people overvalue quality instead of quantity.
You can have one prospect who is a 75% chance to be an impact player in the majors.
If you get four prospects who have 50%, 30%, 20%, and 10% chances to be impact players in the majors, there's also a 75% chance that at least one of them will be, plus the 15% or so chance that more than one of them will be.