> The other key is that the hordes of people should be expendable.

Except that they really weren't.

The Egyptian pyramids, for example, had an entire support ecosystem around it--guilds, hospitals, lodging, everything. The people working on building the pyramids were probably slightly better off than the average person.

We have several recent examples of countries that worked their populations to death. Their GDP was in the dumpster.

Those methods cannot compete with free labor.

Pulling on this thread a bit more - slavery died out because it became uncompetitive. I suspect it being found to be immoral around the same time is not a coincidence.