I'm not a terribly regular poster and don't 'live' on Board but I have thought about using to pose this same question. I was active in the late '70s and very beginning of the '80s. OP mentioned 15 minute 5ks, but I remember sub-30 10ks regularly at small-time events that attracted no out-of-the-area talent at all. I ran a 20 mile race back then and have done it recently too, and did 2:11 and change one year. I was a teenager and didn't get top-3 in my division. I don't remember the overall place since I was so far back (50th?) they there was a sea of people not only behind but in front of me too. One recent year 2:12 was good for second place overall. I made it around 10th place by breaking 2:30!
I don't think the sun-elite category has disappeared but rather one step below that. The guys who do 2:13 marathon still exist, although you can count them on your hands in the US, but the guys that won the random local races in 30 for 10k, 50 + for 10miles, under 2:00 for 20 or under 2:20 for the full don't. And the next 30 finishers don't either. Not everybody was local- hero status, but there were a whole lot right behind them that would be winning easily these days.
My old17-and-under category has suffered particularly. Breaking 3 never got me a top-3 medal. If I could make myself 15 again I'd go from not making the teenage podium to outright winning. Societal pressures now prevent teenage road racing (it's OK to not run but if you do you'd better do it for the school) but not everyone in front of me was in my age group. Maybe 5 were, which leaves 30 older folks who were under 34 in a 10k in a big race or 40 under 2:52 in a marathon. The depth at amateur levels was shocking compared to now.
One race had a T-shirt for a sub-hour 10 mile. They actually gave away a significant amount even to my age division. Then they stop even making the shirt. I know what happened with the kids - just look at the one young lady in recent American history who has run marathons and the ridicule she received - but even the good 30 year olds seem to have all but disappeared. Tragedy in my mind but good because I am still competitive. I have aged and slowed down but so has everyone else!