Actually, Americans WERE better at longer events during 'the Boom'. You suggest that today's runners have smashed old records at every level from high school on. True, in a sense. The 5000 records have come down. Your statement is NOT true at all distances. Admittedly, you do write 'except the marathon'. Popular high school and college training is great for the 5000. Nate Jenkins wrote regularly about this. He suggests that the training commonly done in the US is perhaps the best in the world for distances around there, perhaps even 800 - 10k. He goes on to point out that this is not true at the marathon.
I'm not sure we disagree at all points here. You seem to acknowledge that at the longest commonly contested international events, something is wrong. The high school and college systems are. Of course, and a couple other posters confirmed my claims that kids used to run road races, these are optional. My training group didn't join the track team. I never met the track coach. Apparently a lot others did the same as we did since 10ks and marathons both were loaded in the 17-and-under division. I remember one time going under 34 minutes and not getting top-3. Again, didn't have to even wait around for the awards!
So, and let me reiterate that other posters (likely of my generation) have agreed, you don't HAVE to 'run track.' However, peer and parental pressure make it essentially mandatory now. Add heavy-handed recruiting by coaches and it would just be socially unacceptable for the best distance runner in the school district NOT to run for the glory of the school. Imagine me at Loudon Valley. Coaches who actually have the nerve to contact parents would have ruined my teenage running 'career'.
I walk in after school. My dad says "That track coach called again! She says you haven't shown up for a practice yet!! Why can't you just get with the program!!!" The result would have been me quitting the sport if I wasn't really allowed to do it on my own. No tragedy for the record books, but I'm at it 40 years later, so for me personally, it would have been a bummer to feel I was shut out by societal pressures.
So, what's wrong here? US distance times have been great up to about 10k. But not longer than that. Rupp hasn't been able to get past Hall's half time, and has beaten his record-eligible full by 10 seconds. If his shoes actually ARE worth a minute at the half half and 2 at the full (I'm not convinced of his, but Rupp's fans seem to believe this)then he is quite a bit slower at both if both guys were in similar footwear. And Rupp is arguably at the top of the heap right now. I mean, he's good but why aren't a bunch of guys close to 1:00 and 2:07, which would be consistent with where regularly occurring US 5k times are? Ritz and Hall both were, and they're in the 'heavy training while young' camp that I believe Sara was referring to.
About the "East African Invasion' of world distance running. I don't disagree that it was just starting during the time period that I and a few others have referred to. We don't disagree there. This would be a relevant point if US marathoning had improved to where 2:06 had become common and an occasional sub-2:05 happened. In that case, yeah, we have gotten better, and American Records HAVE come down, but these other guys are regularly a couple minutes faster. In reality, that didn't happen and guys like Bjorklund would still be right near the top of the heap even today.
If shoes have actually become a minute faster, plenty of '70s and '80s guys would possibly be in the top-3 in Atlanta. Joanie in her prime may have won. It's true that Kenya has changed the world, but we really haven't improved much during that time. Will either US marathon record be broken? Almost certainly, but unfortunately, maybe not by anyone active now. Somebody please prove me wrong, but I'm not holding my breath! On a very closely related note, will the US high school boys 10000 record, as recognized by T&FN, ever be broken? I think it's not coincidental that a record set back in the days we old guys always glorify is untouchable for 40 or 50 years. Yeah, we (the US - not necessarily me personally) actually WERE better back then - at long distances.
About your last paragraph. How old was Wanjiru when he first broke the half marathon WR? The leading lights (of which he was one) of world distance running don't wait around until 24 to 'move up'. Are you sure there's no reason to up the mileage and possibly even get into the marathon by your late teens? On thing I'm positive of is that the next WR holders in the half or full won't be anyone who spent their teens in races of less than a mile.