If you take a look - overall population may have grown but High School pop shrunk starting in the mid 80's and then started to get larger in the late 90's -
Here is year by year:
https://nces.ed.gov/programs/digest/d17/tables/dt17_105.30.aspBut to summarize in 1969 high school age kids numbered 13,037 (thousands) by 1979 there were 13,616. Then it started going down by 1985 there were only 12,388, 1990 11, 341 - then it started to go back up and by 1995 you had 12,502 then in '00 it finally reached back to 13, 517 and you got back to where it was in the 70's . It continued to rise during the aughts and peaked at just over 15M in '07 went down a bit till '17 and up now over 15M and is supposed to stabilize for the 10 years or so at mid 15's.
So if you look at the trend in times - esp in distance, high school aged population is certainly not the only thing but it sure is a major component and what I would start with if I were building a regression equation to predict times. Also since a few meets provide a disproportionate percentage of the times - the next thing I would look at is the availability of meets like Arcadia where kids go fast as we all know that its a lot easier to go fast when everyone else in your race is doing same. With travel much easier more meets like that will help greatly with the depth.
Now long jump does seem to be a odd outlier but it was also dominated by a few statistical oddities and that may play a part too. Not sure about your contention on doping helping in LJ or not but if it were you would expect (much like you see in the weight events) that the 80;s and 90's would dominate the lists as drugs were far more invasive during that time.