My problem with this statement is how do you know when someone's peak value is? your logic is 100% sound. But doesn't peak mean that value will go down? What information could you have about a guy that he's about to deteriorate that other teams would not have? aside from hiding medicals.
We can guess, and sell prospects when their value is high, but to assume peak value would be to guess they're going to regress. If we did that with guys like Mookie, Bogaerts, Vazuez etc. The return would have been a fraction of the value they have provided for the Sox over the years.
I think it's all about balance, the purpose of the farm system is to supplement the big club, and sometimes that means holding your chips and other times it means trading them away. I don't necessarily believe that trading a guy away means you believe you're trading him at peak value either (not saying you said that just saying it for conversational purposes).
I think G.M.s sometimes trade away guys that they're legitimately sad to see leave their system but they traded from a position of strength for a team need. With that in mind, there is a hypothetical scenario where you trade one of and keep the other of Dalbec/Casas.