I'm going to have to agree with the other people who have commented so far. Music has never really been a middle class job. But I'm going to disagree with the entire premise that streaming and piracy have killed the middle class musician. I'm going to argue that streaming and piracy created the middle class musician.

Are there people living middle class lives in the music industry? Sure. Probably more today than ever before, and streaming (and possibly piracy) has a lot to do with that. Musicians don't make money from album sales, they make money from live shows. The middle class musician tours constantly because that's their full time job. It's not a 9-5, and you're not home with your family at night. I'm absolutely positive that this is the reason more people don't live middle class musician lifestyles.

Vanished? Nah. Never existed, at least not in any significant way. It's only now due to the Internet that small time niche musicians can make any money and get any sales without major labels. And major labels aren't in the business of making middle class musicians, as Jaron Lanier pointed out.

I'm also going to detour to your source... Jaron Lanier is famous for writing the story "Piracy is your friend", and a few years later writing "Pay me for my content". I would take Jaron Lanier's music industry opinions with a grain of salt.