I have mixed thoughts on the firearms part. While largely agree certain things I feel are inaccurate.

First off, firearms were spread because they are cheap. Crossbows gradually phased out because they were so complicated and expensive. Bowmen required people who were practically born archers. Both of these weapons were approaching their zenith while firearms were at the beginning of their development. You could have monstrous longbows with pointed arrow heads which could threaten even fully armored soldiers but you also required monstrous people to operate these. You could have huge and complex arbalests to take care of anything in your way but they were slow, heavy and very expensive. Guns are just a metal tube filled with gunpowder and a lead ball.

As for crossbows requiring lots of training? I have no source to rely on but I am seriously doubt this. Crossbows are the predecessors of guns. Unlike bows their aiming method is far less up to feeling and guesswork. Contemporary guns require to strictly follow several steps and safety measures to operate. Gun drills are the origin of modern military training. The difference why crossbows required more skills is within the doctrine sprouting from the nature of these weapons. Crossbows are relatively low velocity projectile weapons which maintains most of its energy even near the end of its path but for long range shooting it has to be angled accordingly. Guns are high-velocity projectile weapons shot in a straight line which get weaker with range. Crossbows are more expensive thus comparatively rare. Guns are cheap and used in masses.
Crossbowmen were often prestigious mercenaries by this time. Gunmen were a mass infantry who were made out of peasants under a relatively short time.
You could say the whole myth of muskets being useless in anything but volley fire stems from here. No, their first use as the Handgonne was as siege weapon designed to shoot down castle defenders or in reverse. During sieges range is everything so you can bet they didn't just stand right next to the enemy's walls. Neither they were lamenting "Oh no, they shoot with arrows! There's no way we can attack them from that far!". Seriously, if anyone had a weird idea like this, just forget it. Compared to modern guns the muskets were sure less accurate. Black powder is weaker, musket balls are heavy and not even tight fit (which is no error, there are several advantages to this outside accuracy) and then there's the obviously lackluster quality control compared to nowadays. They had issues with accuracy but so did any other weapon. Bows and crossbows were no better. If anything contemporary tactics often preferred to angle both these weapons by 45 degrees and send volley of bolts and arrows as far as they could.

Okay, back to point. Muskets at average were fired from 30-50 meters. Why? Because that's how their doctrine worked and it was undoubtedly efficient. Here's a few points why they needed to do this:
1.) Muskets are direct fire weapons, any attempts at angling their bullets was unpredictable
2.) Musket balls grow weaker with distance 30-50m was the best they could be any use against armors (well, the non musket-proof kind, at least)
3.) Musketeers had far less trainign than any traditional ranged combatant in the past.
4.) 30-50 meters was the optimal distance where hundreds of guns firing at once could fill out most of the area in front of them with lead.

So yeah, this has nothing to do with people suddenly going stupid and opting for an inferior weapon. Neither the truth is "Gunz too OP, plz nerf". Guns, bows and crossbows all had their advantages and disadvantages. If we are at it let's briefly crush a myth called as "firearm phobia". This is a curious thing in fantasy where authors avoid guns because it'd render armor meaningless.
The reality was pretty much the opposite. Plate armors were partially made with firearms in mind. That and because chainmails were too cumbersome and expensive (yup, that's another common mistake of medieval fantasy, you should look it up). Plate armors pretty much existed at the same time as guns and we already pointed out that people weren't stupid during that time, either.

That is because while contemporary muskets had a chance to pierce armors from 30-50 meters this wasn't an universal fact. Actually for a truly killing blow musketeers had to let armored soldiers as close as 15 meters. Basically just barely enough to fire off a single volley. Of course this rarely happened for obvious reasons so for the rest of the medieval times heavy armors actually did their job quite well.
Of course as time went on the muskets improved, got bigger, better designed and used more powder. Coupled with their improved reliability they were an increasing sight in combat. Still, people designed armors which can handle these. This is what they called "musket-proof". Such armors usually combined two sheets of different hardness and pressed together. While such method was better than just thickening the armors it still increased the weight somewhat. In addition crafting these was an art which not everyone could do well. So yeah, with armor no longer giving a reliable protection by the 16th century full body armors gradually went out of fashion. That is because unlike the common perception knights aren't stupid to wear armor they can't comfortably move in. a medieval knight's equipment weighed about half as much as a modern soldier's and it mostly involved the plate armor with its weight distributed evenly on the body. Also while after a certain point full body armors were no longer practical the breastplates and helmets never truly died out and technically we are using these even now, even if they underwent tons of improvement.

So yeah, next time please avoid freaking out when your players wish to build muskets in your "pure medieval setting".