"Because all the classic Civilisations and Empires needed lots of dumb workers and peasants who were able to do their simple work, but not smart enough, to organize any opposition against the current empire."

I think that's a pretty preposterous claim. Selective breeding of humans has happened in earnest very rarely in human history. Far too little to have created an evolutionary bottleneck. The Flynn Effect would seem to indicate that the opposite has been happening at least in recent history.

https://en.wikipedia.org/wiki/Flynn_effect