Mainstream culture was always bland, safe, and predictible. We remember auteur cinema from the 1970s for example, because they stood the test of time - the great works of Coppola and Scorsese, but the top grossing films of the era were no better than the MCU of today. The same about TV. And music. Look at old Billboard charts, at old Grammy winners - none of them represent the "best" of any decade that we look back at with fondness.
Are Alternative weekly newspapers disappearing. Of course. But that's not because there is no outlet for those alternative voices anymore - they are just not being expressed in a dead tree-based medium anymore. They're on YouTube, on blogs, and yes, on Twitter. Do they have the same amount of reach as late night show clips? Of course not, but that was always the case with alternative culture - it was never as big as the mainstream.
The controversial (for this site) reality is that Alt-right and ALt-left are the counter cultures. Mainstream American media is extremely centrist and safe, and always has been. The US political system and judicial system are extremely conservative and take very little action except to repress civil rights. The "cancel culture" the right freaks out about IS a counterculture. A perhaps overly left-leaning, but a counterculture to the mainstream nonetheless.