The Mets are in a very strong negotiating position. They have no pressing, urgent need to trade Dickey at all, nor do they have to negotiate a contract extension with him right now. They have a hot commodity that's very much in demand.
Alderson is right to ask for the moon for Dickey. If he gets it, it will serve the Mets well in the long run. If he doesn't get it, he can:- use that as leverage to negotiate a good deal on a contract extension for a player who has already stated that he'd prefer to stay in New York;- try to trade him again at the deadline when other teams may be more desperate for a front line starter, or- do both.
So far, I'd say Alderson is playing this exactly right.