> Indeed, there are fewer commercially successful artists

Source?

I reckon every decade had a handful of artists dominate the charts, so it wasn’t unheard of at all to see multiple chart toppers by the same artists at any given time. In fact, given the gatekeepers of popular music largely were concentrated around radio DJs and the “industry machine” of record labels, I’d hypothesize there’s a larger on average number of unique artists in the top 100 since the 2000s than before.