“as financial aid has improved at the top schools and their proportions of first-gen students have increased, these schools are admitting more and more kids for whom they’re both the best and most affordable option, and yield ticks up as a result”
A very sensible and logical hypothesis.
On the other side of the same coin: the proportion of non-first-gen students have decreased, and the competition among this population have increased. As a result, in the past, a super strong non-first-gen student might be cross-admitted by 2 or 3 of HYP, but today the chance of being cross-admitted by 2 or 3 of HYP is probably lower, thus leading to a higher yield for all of HYP among this student population.