I don't have a lot of time for detailed analysis, but this blog post by the Dean Dad is on to something, yet might also miss something: Competition for top colleges is simultaneously getting more fierce, and students seem to be getting worse (as measured by need for remediation and similar things). He analyzes it in terms of inequality hollowing out the middle so that the worst get worse while the best get better, but I'm not sure he's right. I'm sure he's right about economic inequality driving the competition at the top, but it could easily be the case that students are simultaneously piling up more tokens of high achievement (fancier resumes and whatnot) AND are getting dumber.
The tokens of achievement are not themselves achievements. I've seen too many students who took AP calculus AND tested into low-level math. The institutions place an emphasis on producing students with the word "proficient" stamped on paperwork. In 9th grade the students take a test written at whatever level and/or graded with sufficiently generous partial credit so that everyone can be deemed to have passed math. The same happens in 10th and 11th grades, and by 12th grade they are in a class with the label "AP Calculus" stamped on it. Now, that doesn't mean that anybody actually LEARNS calculus, but it means that just as Soviet factory managers would lie about meeting quotas, so too the educational bureaucrats can produce paperwork saying that amazing numbers of kids are taking calculus so #STEMPIPELINE!!!! Now, of course, when they take the AP test they do terribly, but we're assured that that means they just need a calculus refresher. Well, then they fail the remedial math placement test because there are deficits that were never plugged years earlier.
So I see no inherent contradiction between evidence of stiffer competition and evidence of less learning. In fact, I've argued before that students might actually learn more (and you might learn more about them) if they are given the option of low achievement.
Also, they say that nowadays there are kids at top schools who basically started their own non-profits in high school, in order to build a resume for a top school. One of them famously/infamously got into Stanford that simply consisted of the three words "Black lives matter" repeated over and over. Even the most truthful slogan is still just a slogan, but he repeated it and got in because people liked it. If you look at the non-profit he started, it's not clear what they actually do besides publish articles on social media. It's not clear how many people they help. Maybe on some level that shouldn't matter: The kid is clearly a hard worker who knows how to communicate and network. On that level, maybe he shouldn't have to go to college at all. Maybe he should be deemed ready for white collar work. I mean that quite sincerely. On another level, maybe it means he's ready for advanced study because he can clearly excel at things that require lots of time and lots of writing and persuasion. However, what he's really demonstrated is a set of skills for the workplace rather than real achievements that help others, because it isn't clear what he's actually done to help anyone. That's fine, but the institutions clearly want to persuade themselves that they're admitting the kid because of his humanitarianism rather than his career skills.
And nowhere in here is any evidence that anyone cares about deep scholarship and learning. :(