Let's take it from the top but I want to predicate this post with a statement -- please do not fall into the common trap of believing those before us were somehow dumber than us or less enlightened. They were the same human beings we are today and they would not have done something that was clearly worse. They weren't idiots. Now let's dive into this?

Okay, why was gunpowder used? Well early on it was not for guns but for cannons. We can not question the effectiveness of cannons compared to more Medieval siege weapons like the trebuchet. Cannons and mortars were simply better in every way and turned sieges that would normally last a few months into ones that would last a few days. So gunpowder technology would begin development with practical warfare use in the late 13th and 14th centuries and would have nearly 200 years before we would begin seeing personal gunpowder based weapons used in large amounts.

So why were personal gunpowder based weapons, ie: muskets, introduced? One of the common things said is that the longbow was 'better' or that they just weren't effective enough to withstand the enemy just running over and killing you. This is true. That's why in the late 14th, 15th, and 16th centuries the principle tactic was not to make them tactically independent formations but to integrate the muskets with the pike and swordsman formations. The muskets decimate cavalry and enemy "melee only" formations with one or two volleys before engagement and the pikes clean up the rest and keep them safe. The Spanish Tercio is the common point of reference for this as they perfected this formation.

Why were they used though? I mean surely 'better' alternatives existed? Well sort of -- but the principle reason musketry took over so fast was because of ease of use. It took a lifetime to train someone to use a longbow or to become a horse rider. You lose 1000 trained archers and it takes many years to get men which can be considered trained enough to replace those lost. You put a musket in a mans hand and give him a 1-2 weeks of training and he's ready to be put out in the field. It doesn't require physical finesse or super strength or a lifetime of training to learn. This would allow much larger armies to be made and we would see armies going from just a few thousand men at most to being measured in the tens of thousands pretty regularly.

We tend to have a pretty presentist view of history. We see where rifles will be eventually; the average ones for the U.S. army these days has an operational range of 600-700+ yards and we have rifles that have an accurate operational range of almost two miles. So when we look at that we tend to impose those standards hundreds of years ago and go, man, how did they even use those? Well at the time they were revolutionary! Consider the psychological effect of hundreds of muskets (and these aren't modern rifles again, they are crude and therefore loud with lots and lots of smoke) going off at once in your face. That can not be overstated enough. It has been argued that the psychological effect of early musketry was what set it above its competitors more than anything else.

By the 17th and 18th centuries we have musket technology rapidly progressing. It's becoming lighter, more reliable, and more accurate. They go from being able to shoot 1 at most a minute into shooting 3-4 a minute consistently. They would begin to shoot reasonably accurately at 150-200 yards and with volley tactics would make an independent musket formation incredibly deadly. The death of 'melee weapons' as you're traditionally thinking of like pikes and halberds would come with the bayonet. Why slow down these rapidly progressing weapons with burdensome pikemen? When you put an early modern bayonet on a rifle (and I've handled these personally in the past, they are much bigger than we think) you get a similar range to a spear on your musket.

So now not only do you have a gun, a gun that shoots pretty damn fast and causes massive psychological damage and casualties to your enemy before they even reach you, you can just attach a bayonet and get your own pseudo-spear to contest with enemy cavalry and pikemen yourself. You may not be as effective as those 'melee troops' but the advantages gained by having hundreds of musketmen getting 1-3 shots off before combat would even begin offset that more than enough.

In a way I would say 'melee weapons' were never abandoned or replaced but just experienced a period of change. Bayonet charges would be a principle tactical maneuver for hundreds of years even into the late 19th century and World War I with effectiveness. So we're talking from the late 14th/15th century to two decades into the 20th where "melee combat" and weapons were still a central component to Western warfare until it would be overwhelmingly replaced with the power of guns. So a five century transitional shift sounds a lot more reasonable than an overnight abandonment of pikes for muskets, huh?

To be precise, the rifle- so named from the rifling of the barrel which dramatically improved the accuracy of rounds over traditional smooth bore weapons- didn't replace melee weapons. Nor did black powder weapons. It'd take hundreds of years to go from the earliest examples of firearms reliable enough to be employed on the scale of a military, and not just as an oddity, to the modern implement which has dramatically removed the role of melee weapons.

It would not be till roughly WW1 that the fire arm properly replaced the melee weapon because until the advent of the machine gun and other sufficiently portable, high volume weapons, guns always had a flaw, and if it wasn't compensated for with units of pike-equipped soldiers, it was compensated for by issuing bayonets to infantry. Even today bayonets see service. And knives. In WW2 elite infantry divisions such as British Commandos were trained in the usage of knives. More modern elements such as Navy SEALS or US army rangers are also trained in martial arts which, while not a melee weapon per say, still illustrates the fact that firearms are not the dominant be-all, end-all of combat.