I didn't really state my whole view, but hard exams like Oxford and Cambridge use make sense for elite colleges to use (rather than unrelated extracurriculars), but reforming the whole education system to be oriented around a single high stakes test like in China or Korea has its own severe costs. I do not want high schoolers to spend 20 hours a week at hagwons, and the current resume filler system is also terrible. I do not think declining admissions rates at elite universities reflect that the students are any smarter or more prepared than they were 20 years ago, but rather they are much more cutthroat about many things that are orthogonal to being successful adults.
Ideally we'd follow a more exam focused system more like the UK, though I wouldn't want to require all students to only study 3-4 subjects towards the end of high school. But something in the european model of IB/Abitur/A Levels, where there are serious exams in various subjects at the end of high school for all uni bound students, plus some special higher level exams for the most elite unis (in the vein of Cambridge's Sixth Term Examination Paper). We could probably repurpose AP exams to fill a similar admissions role to A Levels, and possibly use the AMC/AIME/USAMO more explicitly for admission.
Edit: this doesn't let me reply again, I think the chain got too deep. But the point is that we're not just using AP exams and USAMO scores, we're also using a pile of other metrics around extracurriculars, GPA, and honors societies, and the end result is stressed out children and not actually having better outcomes than we did two decades ago. Declining admissions rates at elite colleges do not reflect smarter and more productive incoming students.