I think the problem is that the longbow is not the only improvement in archery technology. Classical era societies had many fine archers in drier climates where they used laminated bows. These didn't work well in wet climates though, as they would fall apart in battle. I doubt that is possible to replicate in Civ. The longbow itself was a Welsh creation, I believe, and adapted by the English. It was one of the stongest weapons of the late medieval period. The crossbow, by comparison, was an early to middle medieval weapon. They are each designed for different purposes. Any peasant can easily be trained to fire the crossbow, which is slow, but accurate and deadly. One later improvement was made with a pavise shield, which allowed the crossbowmen to turn their back to the battle with a large shield protecting them while they reloaded. The longbow takes an very long time to master. English kings required their subjects to train in the longbow from an early age.