1) Jesus' prophetic reference to the destruction of the temple in Matthew 22:7; 24: 1-2.
2) The assumption that Matthew and Luke used Mark extensively as a source. ( I agree with this one, but keep my seat belt on)
3) The assumption that it took decades for Matthew and Luke to do this.
While there are scholars who date Matthew and Luke earlier, they are few. I find this to be disappointing on a number of levels, the greatest being that it doesn't seem smarter to me. While I would agree it is likely that Luke and Matthew both utilized Mark (as well as eyewitnesses and other written and oral sources) it is not necessary (or to me probable) that it would take 30 to 40 years for Matthew and Luke to do this (Most scholars would date Mark to the late 50s or early 60's AD). This assumption is often based on a theory that Matthew and Luke were written by communities rather than individuals (a theory that we have NO evidence outside of theory to support). I would agree with Martin Hengel, R.T. France, D.A Carson and others who would assert a more realistic relationship between the Gospels.
Regarding Jesus' prophetic teaching in Matthew 22: 7; 24:1-4, I am reminded of my post on naturalism from last week. If a scholar is a believing Christian, why would it be difficult to believe that Jesus spoke a prophecy that came to pass. Why would we attribute this statement to Matthew's apologetic, without the historical evidence to substantiate such a hermeneutic (some believing scholars have made concessions like this one so that unbelieving scholars could in good faith affirm their exegetical skills). I, for one, cannot reconcile such a naturalistic worldview with my faith or my mind.
Now, I'm not saying that Matthew and Luke could not have been written after the destruction of the temple, nor am I trying to marginalize friends who would disagree with me, but I do want to challenge students of the Bible to think for themselves and to not be afraid of challenging the long-held assumptions of the guild. This is how we progress. Fresh minds and new insights keep us all honest.