And then someone invented the wireless and the record player. According to the myth, it quickly reached its optimum time when some fashionable person was in their youth. And it has been degrading ever since.
Now, as for me, I would think all commercial music is worse than all traditional music. If you can talk, you can sing. But here someone in this thread has suggested that perhaps as little as 1% of the population can sing well enough to be autotuned! (This is insane. The proportion of the population who can sing well enough to be autotuned is much closer to 100% than to 90%.)
Someone else might say that commercial music has done quite well, and it keeps doing better. Certainly there's more and more detail added to it.
But the fashionable story is that at some point, probably in the 1960s, music became brilliant, and then all these grubby gen-xers and millennials got their grubby hands on it and turned it all into the most dreary computer driven sounds imaginable.
Even before that, you had music halls as popular entertainment (and music hall music was indeed very formulaic, arguably more so than modern pop music).