Here's a potential alternative.
The Vietnam War really started escalating in 62, with deployments tripling in 63 and at a peak in 68. Going backwards 17-18 years from when the HS list fell off a cliff (in 84 on the track according to a post above) you end up in 66-67. War ends in '75. 17-18 years after '75 sees arguably the worst 2-3 year period in the history of US running.
Once your running program (at a micro i.e, HS or macro level i.e. nationally) is in the dumps, it takes time to build it back up to what's possible. They say life cycles go in 7-8 year waves and it typically takes that long to build up a solid program. 7-8 years brings us to 2000. Now the HS'ers start running fast again. Meanwhile, post-collegiate guys are still those who were unfortunate enough to go through HS in the 90's so 2003 will obviously not have depth.
2013 is about 6 years after the HS lists started exploding, meaning a lot of those juniors and seniors are now really coming into their prime. Given that 2013 was very deep for US HS'ers one would expect 2019-2020 to be really strong at the post-collegiate level.
The War on Terror started in 2001, meaning that just as the current crop of HS studs hit their prime, those who were born in 2001-2003 will be the juniors and seniors in HS. Will we see another dip in performance? The Vietnam dead (58,209) and wounded (153,303) is substantially higher than the War on Terror's dead (6,717) and (50,897), so my guess is no.