Can someone explain to me why some people think it would be more profitable for the super-rich to replace everyone with machines, rather than continuing to exploit the very people who buy the goods in the first place?

Goods are produced for consumption, and for that, there must be a consumer. All this advertising is literally designed to prompt you to buy a product that, in reality, you do not need.

Why, after a whole history demonstrating their real need for people to constantly buy things, do they suddenly abandon that model the moment they acquire AI? Sure, they could live in affluence thanks to robots, but they are human too, and they also need a sense of forward momentum. That drive used to be about earning more and more, pursuing growth that couldn't exceed the purchasing power of everyone else.