The Beato cry was about the decreasing lyrical poetics content quality in popular chart...
Not about genres, not about tastes, not about nostalgia...
But it ask too much for people to think about why this phenomenon even exist
Fair enough, allow me to take a stab or two
1. As has been pointed out previously, the zeitgest has moved on (can we say reversed itself). The 60s / 70s were a time of youth, newness, daring and creativity. You were not to trust anyone over 30. Heck, you were not to be over 30.
In the present era risk management reigns supreme, and what's safer than proven sellers?.Hence New Beetle, New Mustang, 80-year-old rockers selling out arenas all decked out in ADA ramps, the Marvel Universe, Mission Impossible 12, McIntosh amps, whatever.
This arc moves in unison with the XXL-sized boomer demographic passing through the boa constrictor of time.
2. What was once culture is now known as entertainment, and Internet has made possible its present, extreme granularity. This is rather a good thing in some ways: without Internet, many of us would have never known many of our favorite musical artists, and with the Internet there is no way that talentless British bands could ever turn the wholesale appropriation of Black folks' music into billion-dollar careers. If the Rolling Stones came up today, they would hustle to book gigs in half-empty taverns.
The downside is that a shared culture and a common language have been lost. Whether you loved the Stones or laughed at them, you knew them. Were you a rocker or a mod? Stones or Beatles? It's probably safe to say there is no equivalent today. Not that we ran out of reasons to fight each other, but that's a different conversation.
That beato character appears to be a bit controversial. Perhaps some folks reacted to him rather than to the questions you posed, which are very good questions indeed.