I don't find this type of explanation convincing at all. If your claim was true, then just look at the fact that we still can't reproduce such structures today. I would allege the type of sentiment and thinking your paragraph is representing borders on arrogance. I'm reminded of the story of the illiterate Roman soldier w ho murdered Archimedes and said that man was just drawing circles in the sand.
We are a violent illiterate people who murdered Incan Archimedes and their khippus and since we can't understand their science we try to put down their structures as 'throw hordes of people'.
The other key is that the hordes of people should be expendable.
Imagine what our GDP would be like if companies were still allowed to work us all to death!
Oh wait.
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.
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.
Most of the stuff from the ancient would is quite easy to reproduce--throw hordes of people at it. The ancient world didn't have some fantastically superior knowledge.
I forget the magician, but he stated something along the lines of "Magic is simply being willing to devote to accomplishing a task an amount of time and preparation that any rational person would regard as insanity."
Sadly, when the ancient world couldn't support throwing hordes of people at the problems, it collapsed.