Both arguments for and against are very compelling and on their own merit, it is hard to pick a side. However, someone said "never say never", and I think this is one of those situation, where if running is going to remain a human sport for the next hundred year, it is inevitable. Whether it is technology, new training methods, or we humans have physically changed since the 1900s, there has been gradual improvement in the marathon but for some reason, people believe it has ended in 2013.
There are several things to consider in the marathon and in any sport in general. One thing that has been already mentioned is the course. Each marathon is unique based on its course and this is the major determinant of time. Interestingly, course records vary in the order of minutes between among courses. Do you believe there is no newer course in some random country that will ever be made that is faster than say, Berlin? Secondly, we can not rule out that a freak of nature will not be born in the coming years. Now whether Bolt is drugged or not, his built really makes you wonder what a future sprinter will be like. Look too at Lemaitre, these guys are build differently than the sprinters of yester years; they are lean, tall, and probably have more endurance than earlier sprinters, while still having the powerful bursts. Thirdly, how do we know that we have exploited all the world potential for the marathon? Think about it, before 1960s when Kenyan's started running officially with the debut of Keino, probably not many then saw the eventual coming of generations of African runners. Keep in mind that to be able to pursue non-essential human endeavors, you have to meet the basic needs first, and believe me, several African countries have not gotten here. Even in Kenya, world class runners even in the 1990's still did other things on the side (usually farming) instead of training all year round like they are doing now (due to the increasing ridiculously high payouts in world marathons). Even today, Kenya has a fairly good 400M potential but our best facilities are laughable for sprinting compared to world standards. I would be very surprised if in the next 20 years Kenyans don't start assaulting 400M. Look at the physique of all 400M guys from Kenya. They are all exceptionally good 800M guys with a fast 400 and that's it. They have lack muscle and they lack technique, but they have fair endurance. While the case for Kenya and Ethiopia is harder to make, think about other Sudanese and Somalis. These guys have had more to worry about than Kenya or Ethiopia but they are now just stabilizing. How do we know there is no other Farah out there. Another example is Botswana but for a different reason. This one is one of those where although all needs are met (Botswana has always been peaceful, democratic, and also well-to-do compared to many African countries), people must subscribe to a culture of running. For Amantle Monsho and Nijel Amos to have such fast times for 400M tells me that this country probably has more. To conclude, as far the marathon is concerned, the human trend over generations has been toward faster times and their is not evidence of stagnation. In fact, the fastest times ever were set only around 2 years ago and by different people!