The typical user using the user agent also wants access to "premium content", and also typically is willing to jump through several hoops to get it. Like paying a lot to cable tv providers, signing up for international VPN, risking downloading viruses, etc. etc.

The danger with DRM in the browser being universally available is that more and more content will get locked behind it(say YouTube), but lets not pretend that content providers have not been willing to forgo some revenue in favor of locking things down. E.g Netflix, HBO etc.