You hit the nail on the head! All it will take to really push down the marathon times closer to 2 flat is a super elite runner like Bekele in their prime to move to marathon. Rememeber, it is only in the last couple of years that really top runners in their prime are pursuing marathon, and that, in part, is the reason for the fast times. Basically, runners moved to marathon after they were no longer top of their game in 5 and 10k. As the prestige of 10ks dwindles and the popularity (and money) in the marathon continues to rise, I am weary of what a young super elite runner will do.
On another note, the word "never" should not be thrown around unless something is close to impossible.