They argue for later dates not because they are good historians but because they have an agenda. The agenda is that these gospel stories were committed to writing so long after the deeds of Jesus life that they are suspect, much like the game of "gossip" or "telephone" from our younger days. It is so vital to them to argue for the later dates that they ignore the "Destruction of the Temple" logic I described above.
"... because they have an agenda."
Exactly.