Interesting claim but I'm not quite convinced based on the data presented. They considered the hypothesis ('myth') that increased popularity of running is what's slowing down the average times by looking at 100th-finisher times and nth-percentile times, but this does not control for an increased number of _races_ being held each year to accommodate all the extra runners.
That's one big thing I was worried about. Also, "We included only races with an average number of finishers greater than 2,000 (for all four distances)" could be a major confounding factor. What if many "competitive" runners switched to smaller events as big events became more "general"? Until this major issue is addressed, the conclusions can only apply to the self-selected set of American runners in large races.
It doesn't need to control for that. The new people who get swept up into participation in running events are invariably the slow ones. Conducting larger and more frequent races to fit them in is neither here nor there.
In a road race held somewhere in America in 1975, it would all have been largely highly fit, fast people from a track and field background.
If more races are held next year to accommodate more total runners, and the new runners are slower, and the number of races a given runner races each year stays the same, the average time of the nth finisher across any individual race would get slower.
Interesting, the goal of the fastest runners must not be to finish highly, if it were, they would distribute themselves to the additional races so that they had a better chance of winning. Instead, the claim is that they each keep running in the same races as they did before so that the slower runners do not have lower finishing places. Wait, not that either since then those new low finishes at the new races would be slower.
The top runners will continue to go to the major races (New York, Chicago, Boston) because they can compete with the best runners in the world, there are appearance fees for elite runners, and they offer better exposure for athletes who depend on sponsorship dollars to make a living.
As for the US Marathon Championships, America's best marathoners usually skip that race (aside from when it serves as the Olympic Trials) because smaller, less prestigious marathons that pay less prize money are often chosen to serve as the US Championship. Why (relatively) smaller marathons? So that your US champion actually wins the race (like they would at the Twin Cities Marathon) instead of finishing somewhere in the top 5 (but not first) at New York.
So I can't speak for every runner, but for the vast majority placing will not be a major factor. I ran a half marathon recently and came in something like 600th - but that number is largely academic I was only really competing against the clock.
That race had several thousand entrance, of which maybe 3 had a realistic chance of winning and another 10 of placing. The things that attract people to races are more likely to be the location (either somewhere convenient or somewhere nice to run), timing (how does it fit in with training) and prestige of the race.
Interesting the prestige of the race means your position is likely to be much worse. But distance races, people don't ask where you came, they ask how quickly you ran.
It definitely needs to be accounted for. In the past decade, there has been a proliferation of smaller marathons. If you made a plot of "100th finisher times" by # of participants, it would be slower for smaller races. If the growth in marathons is primarily on the smaller end, then you'd expect the average to go down.
The article does not take into account race size - it plotted 100th finisher times by year, which is not the same as plotting 100th finisher times by # of participants.
