Another factor is that many of the fast times today are made on fast flat courses (Dubai, Rotterdam, Berlin, Frankfurt, and so forth). If you compare the winning times of the same races in the last 20 years, you will see less remarkable improvement.
the people who believe in the idea of a sub 2 all use the same logic that begins with "who would have thought we would now have guys running..." well let's go back to what the world marathon record was before the advent of EPO in the late 1980's. Use that as a baseline for your logic, rather than what guys are running now that time is around 2:07. To think that current times have not been significantly altered more by "technology" versus advances in training is naive.