Forget Cano. If Diaz is a premier closer for the Mets for the next decade, and a key component of a perennial winner, then it's a good trade for the Mets, regardless of what Kelenic and Dunn accomplish or do not accomplish. Taking Cano was part of the Diaz price tag.
No guarantees of that of course, but then no guarantees about Kelenic and Dunn either.
I feel strongly that trades should be evaluated based on whether the guy you got helped you achieve the objective that you had set when you traded for him. Not on how the guys you trade away do. If Diaz helps the Mets win a World Series, then I don't care if Kelenic is the second coming of Mike Trout.