OP, I think this is a big part of it. There aren’t necessarily a ton more applicants to BS, but each submits maybe 2-3 times more applications than they might have a few decades ago. The same has happened at the college level. And at both levels, there is flocking to the most selective schools. This has made the most well-known schools more competitive while simply getting into a good school may not be.
There are also a number of things that have helped that have resulted in kids being more specialized and having more recognition at a younger age. Youth sports are year-round and more competitive, schools (especially expensive privates) need to offer more, etc. With that said, there is self-selection here, so you may be seeing a particularly accomplished group here (and perhaps some puffery.)
I am guessing that your cohort at CRH had made the most of what was available to them just as kids today are doing with access to different resources. I am older than you and feel like my cohort was very hum-drum compared to these kids, at least on paper, yet many have done incredible things and lived up to whatever promise someone saw in them as the cream of an average crop.
It sounds like you are being very thoughtful about your D’s list AND that you understand well how fabulous or toxic any given school/student mix can be.
If you are willing to go just a bit further geographically than Masters, I will make a plug for George School. Same schedule as CSW, very strong arts, and very nurturing, kind community. It seems culturally aligned with your list.