the expression I’ve heard is that you can say anything you want with statistics if you pick the right numbers… facts
1. more people running
2. more marathons available
3.pecentages are very misleading, as you said they are based on registrants not finishers.
possibly we need to look at total numbers of sub-whatever run in each year by Americans(at races throughout the world), good luck hunting those numbers down.
A. as it is a lower percentage of sub-X may mean more people have money to sent in and possibly not run.. they can enter two and make a choice of target race
B. or so many more people are using the marathon as a health goal instead of just the elite participating
C. after 1986 the fast people, got old & slow and their replacements decided to make money playing basketball, football and baseball
I’m part of this horrible trend, I run slow even though I’m working hard at improving… I doubt I’ll ever even be sub-4, sorry to skew your stats
but I also see it in my son’s high school… kids would rather sit on the bench of a losing football team (and get a letter) than go out for track and work hard… my kid(16) runs, but has yet to train through a summer…
I’ll blame part of it on the general trend towards instant gratification, some on video games & tv… but I keep hoping we all improve, both our attitudes & our abilities