Prior to Usain Bolt, the "experts" said that big/tall guys couldn't sprint. Bolt destroyed 100 years of such stupid speculations.
Now we have another set of stupid speculations about marathon running, almost certainly just as wrong.
Partially true though I think a factor there might be that height isn't a big factor. If height is only loosely correlated with sprinting speed then the typical height of elite sprinters will be the typical heights of the seed population.
As for marathon running they may be short because a shorter physiology is advantageous. Or may be short because the seed population (Ethiopians, Kenyans, Kalenjins particularly) is very short. If they can get taller with nutrition, and that doesn't adversely affect their running, then elite marathon runners may get taller too.
Given the $$$ incentives, we'll see 2:00:00 broken prior to 2020, and by someone previously unknown.
Very doubtful, to go from a 2:06:23 to 2:02:57 they needed to drop ~5 seconds/km, to drop below 2:00 they need to drop another ~5 seconds/km. Yes that's possible (and people can do it over the distance of a half marathon). But even assuming the current progression continues we're looking at another 16 years. If we assume the 16 years got a lot of the low hanging fruit (East African population getting access to elite coaching and training methods) we may start to see a stagnation in times.
But in the mean time, we'll see the world record broken perhaps another 25 times, because breaking the world record by 1 second pays just as well as breaking it by 10 seconds.
Google "Roland Matthes", who milked the system by breaking the world record by the minimal amount as many times as possible.
Well you've got 177 seconds to play with, which means you're projecting a 7 second margin for each increment, the typical margins have been 20-30 seconds though that will likely shrink the lower we go.
25 new world records by 2020 is a LOT harder than 2:00 by 2020. There's no risk of someone milking the system. Elite marathon runners have very short windows where they can set world records and they only do 2-3 marathons a year. even if you go in perfectly healthy and trained to have a WR shot not only do you need an ideal course (ie Berlin), but perfect weather too. For instance Reid Coolsaet has been trying to be the first Canadian to break the 2:10 barrier, and while he had a window of a few years where he was likely fast enough the factors never came together. Either no one else was at the right pace, the weather was poor, sick before the race, injury interfered with training, etc. He's still got a shot but the window is closing.
I wouldn't be shocked if 2:00 takes 25 world records, but if it does then we're looking at 2050 or 2075 to see it happen.