I think fundamentally many of those analyses are flawed, because they always start with a data set per year which is generated by taking the N most popular songs per year (i.e., Billboard's Top 100). This assumes the population count and distribution of songs which you would consider "pop" vs. not "pop" is consistent year to year, which I don't think it is.
What constitutes the top N pop songs each year is becoming more distilled and refined as the industry matures, which means they're all converging onto each other and losing complexity. I also contend if you chart the volume of sales for the whole music industry vs. this tight cluster, the cluster's ratio of single or streaming sales decreases.
Meanwhile there's artists in an increasingly large tail making what you'd probably call "pop" in style but just without marketing push, which are more experimental and less refined, and these become underrepresented in the study.