> and slowly trickle down

Seems like a strong assumption that it necessarily does splash damage beyond a certain cohort. I went to a public HS of 4K in the 00s only AP Juniors/Seniors were concerned with this kind of thing and AP kids were only friends with other AP kids (for obvious reasons); I'd doubt that've changed by 2012.

I think this is mistaken; there was a lot of pressure in the 10s for AP classes

At your school, sure. Not at the one I taught at in the 10s. And from knowing other teachers then, I'd guess the pressure was high at only one, maybe two, of the six high schools in the area.

Many badly-envisioned schools restrict access to AP classes or even AP tests, instead of letting kids follow their passions and meeting demand.

Well that's not surprising, urban public schools, at least in the major American city I'm from, are about figuring out who's a good pencil-pusher as early as possible then segregating them from the rest of the riff-raff until they graduate HS. Most of the cohort I'm talking about likely got invited to a 'gifted' elementary school somewhere around 5th or 6th grade or a 7th/8th grade program attached to one of the top HSs.

I only know this cohort well enough because I was on the debate team which most've them used as an extra-curricular for their college application CV but weren't serious about winning.

“Trickle down” isn’t necessarily a good mental model here. Maybe the high achieving types from across the country and world are increasingly concentrating their attention into admission to a few top universities. So the max effort expended by the top students may be increasing, but the median effort could be unchanged or dropping.