First I thought BRTD was talking about the collapse of White "ethnic" culture into a general or regional (White) American identity, and I was like "okay that's a interesting discussion", then I found out he was talking about a general collapse of a collective American cultural identity and tradition, and my opinion changed to "that's the stupidest thing I have heard this month".
No BRTD the collective culture is not becoming annihilated, in fact the creation of mass media and standardised education have resulted in it becoming less heterogene. A author archetypes can through mass education become universal among a population, while mass media have allowed the spread of things like Black Friday or St. Patricks day.
That's kind of point. Mass media means people are no longer locked into a culture they are born into. They're free to select and associate with aspects of others they want to and simply choose whatever they like best. Your heritage becomes a non-factor.
I think a major problem here is that you may be the least cosmopolitan person on this board (no insult), you doesn't seem to get what makes up culture, likely because you belong to the cultural dominant culture not just in USA but also in the world. As such you have a very superficial understanding of what makes up culture. As example when I meet a Arab I don't look him into his eyes or show him the soles of my feet, not unless I want to insult him, when I meet a Dane I don't know at a bus stop, I don't talk to him no matter how long we wait side by side, unless I have a impersonal question to him ("have that or that bus arrived", do you know what bus I have to shift to and where to get to some specific place" etc), or to complain over the weather (the social acceptable smalltalk). When I go into the bus I don't sit down beside a person if there's still empty seats, where I can avoid sitting down next to a person, I don't talk in the bus and try my best to ignore everyone around me, unless of course I meet someone I know, this is seen as the proper behaviour. If I for example was a Turk I would sit down next to a person, and it would be impolite not to make small talk.
All that is make up real culture, running around playing Ingress, listening to specific kind of music or go into a specific a specific chuch are not usual part of a culture, that's part of a subculture.