You certainly missed/ignored a lot of my points, like say having creatures that have equally deadly abilities, resistances/immunities.
But why wouldn't goblins or orcs have alchemy skills or tools (they are not all stupid)? In a setting where gunpowder is common enough that PCs can hoard it, it's going to be fairly common knowledge and if humanoids can't make it, they can trade for or steal it. And any intelligent creature that has contact with cultures that have ready access to gunpowder will know what it does, how it is used, how to react to it, how to defend against it, etc. Even less intelligent animals can change and adapt their behavior in response to encounters with people with gunpowder or other commonly used tactics and tools (as has happened in the real world). Dragons? They don't need gunpowder. Giants? Aside from hill giants, most are pretty savvy. Hydras? Screw 'em—there are going to be monsters that are vulnerable to various tactics (and if they are using these kind of resources on these sort of monsters, they aren't using them against other foes). Undead, many of them are intelligent and/or have resistances/immunities. Many monsters won't even require gunpowder as they already have abilities that are comparable or more deadly. Never mind the fact that as the PCs level up, they are going to fight monsters that are less and less impressed with their keg parties.
Also, monsters don't need to have the same tricks to stay on and even footing of your proposed scenario (that doesn't even accurately reflect the rules for gunpowder and ignores several facets of the rules). D&D monsters have access to abilities that Cyberpunk (or other games that include explosives) don't, so D&D is on a better footing.
You're the DM—you control what they have access to, what monsters and adventures they face, how much downtime and materials they have, etc. If they want to escalate things, then rise to the challenge of providing them with more challenging encounters and throw nastier critters, adjust the amount of XP the get (if an encounter is less challenging, it's worth less experience), etc.. Or you could talk to the players (heresy, I know) and tell them you find it obnoxious. There are so many ways to address the issue.
There are tools and such for making alchemical stuff faster? Like what? And do, as the DM, just let them have access to them? Sure, you can have multiple PCs craft stuff (if multiple PCs actually have proficiency with alchemicst's tools), but they still need access to tools and materials, neither of which are infinite in availability. Are the tools and materials commercially available? If so, in what quantity and at what price. If not, then the PCs will have to go out and find them for themselves (which, in turn, can be its own adventure). As far as skill checks go, you set the DC, failed checks still use up resources, and failure may have consequence given that they are working with a volatile substance (possibly blowing up or catching the lab on fire on a roll that fails by 5 or on a roll of a natural 1). Given that the rules do not specifically cover the crafting of gunpowder or other explosives, there's a lot of room for the DM make rulings on how it works (you may even require a new, separate tool proficiency for creating gunpowder than alchemist's tools). You also control how much downtime that the PCs have—remember, it takes five work weeks to create 1 keg's worth of gunpowder. And what are people in the PC's community thinking about these gunpowder hoarders—authorities might suspect them of plotting treason and have them arrester and their gunpowder stores confiscated or detonated.
There are a ton of ways to handle these scenarios, you just have to learn how to do so and adapt.
