It feels like the author started by coining a catchy turn of phrase, "premium mediocrity", and then just started writing until he tenuously connected it with his various insights about the psychosocial dynamics of Millennials.

The important detail about "premium mediocrity" seems to be that it is manifestly not, in fact, "premium". It's the end result, the evolution, of armies of marketing people doing their job. A gimmick.

I think the people he writes about are more savvy to this than he gives them credit for. It's just that these kinds of consumption choices are just usually the most convenient, and there are lots of other things to be dealing with.