Top 40 music may be controlled and commercialized but I wouldn’t say that about the music out of the mainstream.
Agree. With the ease of recording nowadays, the selection is gargantuan. The problem is sifting through all the crap. It requires work to find them. As always, the Top whatever lists drive the marketing. Those don't require sifting and are therefore favored by people less obsessed with music. Just go to the Spotify Top Ten and hit Play.
Speaking to your "out of the mainstream", metal has seen a similar negative shift in quality. If you go to Spotify's top lists the metal is either unoriginal, unimaginative, monotonous or not metal. There's even a song called Call Me Little Sunshine. Can you imagine a metal song being called that? Even worse, it has never-ending repetitive lyrics just like pop.
You will never walk alone
You can always reach me
You will never ever walk alone
You will never walk alone
You can always reach me
You will never ever walk alone
You will never walk alone
You can always reach me
You will never ever walk alone
And the even more out of the mainstream extreme metal has gone from having groove and melody to favoring dissonance. It's like they're doing the opposite of pop and trying to be so different that they've left the realm of listenable. So the quality issue may be generational and not limited to pop.