That's not actually what has happened. Roll back 100 odd years, the average age of entry into the work force would be around 14 for males (very useful in coal mines for the narrow bits), retirement was 65, but most people didn't make it that far. The typical working week was 6 days. Now we have a significant percentage of the population studying into their twenties, and at the other end retired and living into their eighties or nineties.

That's the predicted "unemployment". There are probably significantly fewer managers per se, than there used to be - companies like GE used to have hierarchies 40 or more levels deep.