This is not about whether or not this or that particular song is better, is the top (most listened/streamed/talked about) music in general losing variety in all these aspects (harmony, melody, timbre, lyrics, genres, dynamics, etc).

That's not subjective, that's objective, and has been measured to get worse.

The "subjective experience" could be good, the same way people can prefer McDonalds over a wholesome meal by the best chef. Doesn't mean its also good by other metrics, and I for one don't believe individual taste is everything. There are people with shit taste, loads of them.

Even saying someone has “shit taste” is subjective.

But to the degree that we have a culture, though, we also have a non-100%-exact but nonetheless existing hierarchy of artistic works. There might be disagreements, even strong ones, but there's also some general agreement, that not everything is a fuzzy blob of equal value, left for the individual taste to sort or not, and this just for itself.

Is the idea that the Beatles are better than The Monkeys or Oasis, that Aphex Twin is better than Skrillex, that Dua Lipa is better than Justin Bieber, that Michael Jackson is better than Milli Vanilly, in some non-measurable but tangible way really that difficult?

That this, once a common and well accepted idea (related to the idea of the "canon"), appears like beyond the pale for the 21st century solipsistic individual, where only the subjective taste matters, is not really the fault of the idea itself.