If kids were learning high school math just fine 50 years ago (and, as far as I know, they were), then that suggests that advances in pedagogy are either not forthcoming, entirely irrelevant, or overwhelmed by other factors.
It doesn't "suggest" that at all. Perhaps changes in pedagogy were necessary in order to adapt to a changing world.
Education isn't like the naive "encoding/decoding" model of communication, where the subject matter is simply "transmitted" from teacher to students. Even if the subject matter remains stable over time, many other things do not: changes in the media of communication, signal interference (say, from the average classroom size drastically increasing over time), all kinds of changes in the teachers and students themselves, changes in society's expectations of what constitutes success or failure (e.g. rote learning is now widely seen as having many shortcomings), changes in what students actually need to move forward (a career in the trades may well require a much lower standard of understanding in the age of the pocket calculator), ... this list could go on and on. Teaching is not a "solved problem" like that.
50 years ago, high school math at most schools ended with trigonometry. Today a large fraction of college bound seniors have taken calculus, and STEM students have taken two years of calculus. And yet, 50 years ago there were lots of math-phobic kids who, today, are expected to perform at some modest level (50 years ago, they stopped with algebra I).
It is true that 50 years ago a fraction of kids were learning just fine, but more recently the goal has been to make that fraction larger, in a society that actively devalues learning.
