If you talk to anyone under 30, there's a vague sense of 'the past' with a few landmark events - mostly Star Wars, Pokemon, Miyazaki. Beyond that it's all recent comics, superhero movies, video games, and anime, with a big subculture stanning book trends like romantasy.

Most of what happened before 2000 doesn't seem to exist in cultural memory.

It's not quite true that nothing that happened before 1950 exists at all. But you're not going to find many people who are interested in the art, music, literature, design, or architecture of earlier decades - never mind centuries.

It's as a big a break as there was in the 60s. For that generation the 50s were still an influence, but anything earlier pretty much just disappeared.

I guess the sense of a rubicon at the end of the 40s was due to WW2, but why at the year 2000? Because phones? Or big round number effect, perhaps? The year 2000 was built up in our minds as when the future was expected to begin. (Every new gadget produced around 1990 was the Something2000. CarVacuum2000, Ionizer2000, SuperShoehorn2000, etc.)