Its a combination of all things mentioned here so far. Look at hs performances as well. Yes, shoes and fast tracks but i think because of the assistance, kids to pros are motivated to train more/harder to get to those record performance levels. Think about the psyche that results in daily improvements with that assistance. People actually think they are better than Jim Ryun, Steve Scott, Seb Coe etc...at a much younger and inexperienced age. Then the rest of us think these current record breakers should be hauling in the medals when in reality everyone else in the world has the same advantages but are truly better as proven when it counts. Its all about the now but people want to compare to history which is natural.
Carbon fiber replaced wood, sweedish steel, fiberglass, etc...in golf, tennis and track. Once many people got their hands on the tech, history and the way people play the game changes and so to the end results. Evolution.
Still, strange to see 8 kids from the same team run under 4. But, that 8th guy is still the 8th guy on that team and hes going nowhere. Cant score in a conference meet or even make the traveling squad let alone nationals. Just another member of a fast moving herd.