[Waits]
> Rome was already 1 million people during the first millenia, there were buildings with 6 or 7 floors,
Did you really choose an example of a society that used slave labor extensively?
It's hard to make a comparison due to opportunity cost. Imagine you are a person with the innate competence and dedication to eventually become a master craftsman. Do you learn woodworking, stonework, etc. and labor inexpensively for years as a apprentice to eventually top out at being paid moderately well doing work that benefits a single project .... or do you learn to develop cellphone apps that get used by tens of millions or billions of people?
As we create more jobs which are highly leveraged-- where one unit of work potentially produces an enormous amount of benefit all less leveraged jobs are squeezed by the competition.
In cases where high skills and years of training don't matter the effect isn't so great because skilled industrial production jobs don't compete as much for that labor pool. But for these construction jobs, they do.