you are forgetting that there are many players that received help from older members and told them how to proceed fast through the ages and not do the unwanted upgrades at all.
In fact, I do this on a daily basis in my alliance. Players from enlightenment age are advised to only upgrade airport/factory units and specific chapters in library and specific leaders in uni. You have no idea how much time all this saves from them. Armory and barracks units are completely left unupgraded after industrial age. That is the case with mortars, sniper towers, vault, roads, caravans, farms, oil wells, some garrisons, redoubts etc....