Sorry Subway, but you are not connected with the athletics of today, and don't have a real technical focus.
Of course an athlete running 12'49" can earn money running 10 km on road races : this is exactly what they do, and is exactly the reason because they disappear from the track.
Go to see the career of Gebremeskel : how many competitions on track he did after that famous Paris, in the last years ?
In 2013 he ran on track 3 times (13'31" in 5000m, a good 26'51" in 10000m for being selected for WCh, where was 16th in 27'51"). In 2014 ran once on track (winning Oslo DL). In 2015 ran 3 times, winning in Ostrava (13'23"), being 8th in Roma DL (13'00"49) an running 13'05" in Heusden. In 2016, Olympic year, ran 5 times 5000m (3 of them for qualifying according the Ethiopian system), and in Rio was 12th. This year he ran only one 5000m, for him a must, because is part of his contract with Adidas (it was an Adidas meeting).
His activity is now mainly on the road, or in indoor races during winter. This is not the best way for improving under the technical point of view, BUT IS THE ONLY WAY TO EARN MONEY FOR LIVING FOR AN AFRICAN RUNNER.
About your second consideration, I think the top marathon runners of 30-40 years ago could run from one to two minutes faster than their PB, in some competition of today, with very fast course and even pace, inside a big group of runners.
On the other side, this is another point for thinking times about 2:06 can be possible for white runners too, and are possible in clean way, so everybody thinking of doping only for justifying the massive dominance (in the lists) of Kenyans and Ethiopians goes to contradict himself, looking at the possibilities of old specialist, of course in theory.
The third assumption is nor real. At first, according to the Italian experience, self transfusion helped Cova (but don't forget he ran alone 13'18" in Otaniemi before the reinfusion before Olympics 1984, and he was not a good front runner...) but not Antibo, who ran very much faster 5-7 years after quitting the method. In Italy we had some taking advantage, but a bigger number of athletes not able to improve, who finished their career in short time.
This means that there are "responders" and "not responders", the same happens with EPO, and it's a big mistake to think the behavior is the same for every person, in a "mathematic" trend.
About Marathon, NEVER the self transfusion produced advantages. On the contrary, we had athletes who ran their WORST competition in long distances (marathon or 50 km walking) after the self transfusion, and became again top athletes after quitting the method.
Why this difference ? Because to increase the quantity of Hb in the blood can increase the POWER of the engine, but at the same time increases the CONSUMPTION of fuel. So, if (in theory) to have a blood with more Hb can give advantage on the side of AEROBIC POWER, on the other side decreases the velocity of the blood circulation, and doesn't have advantages when the athletes run under the LT, at a percentage of their Aerobic Power.
Also if some of the poster here thinks I'm naïve, or a liar, I well know what I do, and the possibility of INDIVIDUAL effects of training in changing physiological situation. I can guarantee that the limits for clean and talented athletes, always training in altitude, are well better than the current WR. Of course, we need to find top talents, and top talents need to be at top level not only under the physical point of view, but under the mental point of view, too. This is more difficult, when we look at African athletes, normally coming from areas with little education, that traditionally pushes them to do mistakes (like long period of rest, not priorities in their choices, not care when injured, an other factors).
When we find somebody with total dedication, top talent, and continuous focus, we have the best (this is the case of Eliud Kipchoge now, was the case of Haile and Kenenisa and Shaheen and El Guerrouj before).