Crossbows vs. Longbows

[Agema]

Longbowmen worked because the kings of England compelled the male population to train with them (I think) one day a week. They couldn't even be fired effectively without extensive development of the right muscles. Longbowmen had a distinctive lopsided shoulder musculature, I believe.

Further studies have shown that a top notch yew longbow with the right arrowhead would be devastating, but most longbows were cheaply produced with a poor arrowhead, and were not optimal for defeating armour, and generally a crossbow bolt had more force than a longbow arrow. By the time of Agincourt, the French armour was generally good enough to block a longbow arrow at range. You're quite right that the time and effort required to produce longbowmen and their weapons made them essentially impractical when gunpowder came along. A useful side effect of longbowmen's training which may have contributed to their effectiveness is that they would have been at least semi-professional troops, where most nations would merely have raw militia from the equivalent class. Longbows were still used up to at least the mid-16th century (e.g. Pinkie Cleugh), until gun technology got good enough to really make them obsolete.

I'm pretty sure that slings have another problem, which is that they require quite a lot of room to fire - you've got to whirl the sling without risk of hitting your neighbours. You can pack conventional bowmen very tightly, which means you can amass more firepower for your unit frontage, which is a huge plus when the intention is to use firepower to do major damage rather than skirmishing harassment.