Re: the longbow argument (a good 'un). Check out Japanese history, specifically, the Onin War. There's a reason that the Tokugawa Shogunate banned muskets (teppo): Teppo allowed a warlord to drag peasents off fields, give 'em a few hours of firearms care and training, and fling them into battle en masse, where they could mow down the elite samurai with pure overwhelming numbers.
The crossbow is easier to use than the self-bow. The musket is easier to use than the crossbow. The easier a weapon is to use, the more people you can have using them. QED.