Actually - there are way more events now then there was in the 1980's and 1990's. Some 17K in 2017 and has been stated before way more population (best source I found(
https://runrepeat.com/state-of-running
) has it peaking in 2016 at 9 million results globally) other figures differ
But things are getting slower but for a lot of different reasons:
1) demographics - the average participant is older
2) sport maturity - as a sport matures, esp one like running which is more a participation sport, you will get many more slower people as it reaches for new participants
3) Reason for running - Elite competition is not the emphasis of many of these new road races
4) Change in sponsorship - in the 80's there was a some money/shoes going to a lot of people - now its much more money going to very few people
Nobody can look at the college or high school running scene and say competitive runners have gotten slower. The depth of both suffered in the 90's but is now deeper and more competitive then it has ever been. (for example in 1980/81 in the NCAA - there were 5K women runners in XC and 9K in men's XC by 86/87 that way 7K and 10K by 97/98 that was 10.4k and 10.5K by 08/09 that was 14K and 13K by 17/18 it was 15.6K and 14.2K - so despite the loss of programs and scholarships - you have 300% growth in the women and ~50% in the men. High school participation has grown- number wise not % wise - even more than that, increasing competition for those spots and driving competition. (for example in 1980 HS XC for girls was 82K and for boys was 163 - in the latest year 18/19 it was 219K for girls and 269K for boys) - more numbers doesn't always = better times but it it def the way to bet.
It does seem that far fewer are continuing to run competitively after college or Hs though. But anyone who reads these boards could see, why - any effort by just about any one who isn't a WR holder is mocked by "hobby jogger" chants. What would be the point of working toward being a 30 min 10K guy just to be told "congrat's your nearly 4 mins away from the WR" Yet that is where all that fabulous depth came from in the 80's road races. So maybe a little of that blame has to be on us the "fans" of the sport.