The fact that sub-4 skeptics, with their pseudo-science about exploding hearts, etc., were so utterly wrong does not mean that all round-number "barriers" are therefore breakable, and that anyone who suggests they're not will one day be similarly laughed at. When they said 4mins could not be broken, the world record was 4:01.4 and had been reduced by almost 5 seconds between 1942 and 1945-- that's right, during a period in which a huge percentage of able-bodied males of prime athletic age were occupied in a global war. The fact that it remained at 4:01.4 for another 10 years had to do largely with the after effects of the war itself (i.e. much of Europe was in ruin and political turmoil, and had more important matters to attend to than sport). The fact is, while there were some celebrated sub-4 skeptics, many, many more people realized that sub-4 was not only possible but inevitable, because they could point to a number of imminent changes that were going to create the necessary conditions (e.g. reconstruction and the revival of international sport), and because the drop in time involved was so small in percentage terms compared to the rate of at which the record had dropped between '42 and '45. In short, the famous skeptics were largely idiotic outliers who knew little about sport or the factors that drive performance. The record was indeed soon smashed because tons of people who actually knew something about sport at the time knew is was entirely possible, and were making preparations in earnest to do it.
The situation with regard to sub-2 marathon speculation is completely different for a couple of reasons, not least of which is that the percentage drop from the current record to sub-2 is much greater than for the sub-4 on the eve of its breaking. Instead of being able to point to imminent sociological and scientific (legal, that is)changes and advances that will make this mark attainable, most predictors have had to resort to either baseless (and ultimately absurd) mathematical projections, or else to pure speculation (somehow science, or some brilliant coach, will find a way; or, some genetic outlier will appear). What these two tendencies have in common is that they completely ignore the kinds of factor that actually led to (legally) increased performance in the past-- namely, massively greater numbers of people becoming exposed to sport (something that is highly unlikely to be repeated on the same scale in the foreseeable future) and the discovery of certain fundamental principles of human physiology as they relate to performance (again, not likely to be repeated any time soon). Human performance, like any other human endeavor, happens IN REAL HISTORICAL CONTEXTS, and therefore has real limits. Based on what we know about sport in the 20th Century, and about how the world has changed and is changing in the 21st Century, it is more than reasonable to argue that the limit to human performance in marathon running is very likely to be well short of sub-2 hours, regardless of what the math says.
One simple sociological/political-economic difference is that the basic way of life of the great champions of the 20th century-- smallholder and subsistence farming in the highlands of Africa-- is actually disappearing and being replaced by urban slum habitation. It's possible that another way of life that would support the emergence of athletes with both background and the drive to succeed in distance running will emerge, but it's not apparent at this point. I mean, what are the candidates? There certainly aren't any in the developed world. Quite the opposite, in fact.