That is not what he argued. He was talking about upwards of 75% of our population cut.
Also, you have to determine what you mean by 1960s population. Demographics are not equal. Women do not do much for a technological society. They are essential for families, but unlike white and Asian men in their thirties and forties, that is a percentage game. We could have an equivalently sized population as the 1960s, but if half of those men are black and Latino, we'd not innovate anywhere near what we did in the 1960s.
Japan is going to do just fine in the demographic winter because they refuse to allow mass migration of non-Japanese people, Japanese people have an extremely high average IQ, and they innovate better than us where it comes to automation.
That is not our future, however, because of what the Chinese folks call the "white liberal", which I would argue includes libertarians and global8st types who delude themselves into thinking humans are interchangeable components in society, like society is just some machine.