What marked the end of decades?

Despite happening a couple of years later, I sorta feel that 9-11 marked the end of the 90s, the fall of the Berlin Wall marked the end of the 80s, the election of Reagan marked the end of the 70s, the moon landing marked the end of the 60s, and the formation of the Beatles marked the end of the 50s. The end of the 10s likely began in the Wuhan lab.

But it feels like there was no solid transition between the 00s and the 10s. Just one continuous, drawn out decline outside of technological advancements. It's like they're not even their own proper decades.

If Obama's Presidency was the promises of his candidacy rather than an extra eight years of Bush, we could have seen the war waging 00s give way to the peacetime 10s, more or less.

Huge missed opportunity there.