You trade your veterans to accumulate elite prospects. You don't trade your emerging young stars under team control for more prospects. Let's play this out....
If you trade Castillo for 3-4 elite prospects, you will be lucky if 1-2 years from now, 1 or 2 of those prospects end up being as good as Castillo. Do you then trade them as well, for more prospects? Where does this cycle end?
When a prospect pans out and becomes elite, like Castillo, you keep him as long as you can and trade the Roark / Puig / etc. guys for more prospects that might be 1-2 years away.
The KC model sucks. Houston did it right. But once Altuve emerged as a great player, they did not trade him for more prospects. They kept him on the team while waiting for Bregman, Springer, etc. to get to the bigs.