All of the factors you've named play a role in the decline of music journalism. But in the U.S., the problem goes beyond music: All arts journalism is dying (and you can find articles about that if you search online). A lot of it has to do with the death of newspapers.

More than 2,000 newspapers have died here in the past decade or so along with their arts sections. Even the alternate weeklies that used to be fantastic outlets for music journalists (like the Village Voice) are either dead or barely hanging on.

That trend means that after they're out of school, music and other arts journalists have no place to keep practicing their trade or to earn a living at it. They write on social media because it's the only place they *can* publish--but without the editors and mentors who used to help music journalists improve. You're right that commercialism and market factors have contributed to the decline, but responsible newspapers used to help reporters resist those pressures and do honest journalism.

The situation may less dire for newspapers in Britain, but given how Americanized the U.K. has become in other ways, I wonder how long that will last.