My twitter timeline this morning includes seemingly every prospect-gone-bad story out there. From trades that went bust to trades that didn't happen because of untouchable players. Every example is covered.
There is a reason such players are called prospects. No team knows what the player will do in the Major Leagues. But if every team was obsessed with the pessimistic view of prospects that don't pan out there would never be any trades at all ever.
There are plenty of prospects that do grow into capable big leaguers. The trades come from that mindset.