Of course 100% accurate predictions are not possible. Everyone here knows that. Which is why you have to rely on the best way you have of figuring out what the future may hold: probability. Trading away a prospect reduces the probability of future team success, especially if the prospect traded away is one with a relatively good probability of future individual success. The more of such prospects traded, the lower the probability of future success. The exception to this is when you have the financial wherewithal to frequently buy free agents with a high probability of future success. (Which the Twins do not have.) So one must evaluate the increased probability of immediate success that comes with gaining a player and contrast it with the decreased probability of future success that comes with losing a player. In deadline trades the former is almost always considerably less than the latter. Hence, the very reasonable conclusion that most such trades are not favorable.
Logical, not amazing.