You just summed up all my problems with opponents of firearms in a setting. Let's list them in order:
A.) But-but Lord of the Rings didn't have any guns in it!
Yes, and most of these closely resemble that of the early middle ages. If you're such diehard puritan that you don't wish to have anything which defiles such a setting then say goodbye to suits of plate, fancy 17th century clothes, rapiers, longswords and baroque arts you love to add to the setting. If your argument is about that portraying the fruits of progress is antithetical to your setting then be consistent about it. You aren't necessarily need to be sticking to complete historical accuracy but when your actual argument revolves around X being out of place you must remember how messed up that claim really is. Of course the above line about sticking to LotR has a lot of other issues with it, I get to that later.
B.) Guns are boring
And curiously enough other ranged weapons aren't. You can have the same degree of creativity with guns as with a crossbow and actually more. The options for various loadouts alone can keep things interesting. In the end making a weapon's use exciting depends on the writer, not the weapon itself. It's a tool. If you believe guns would ruin the experience then it's your creativity that is lacking, not the fault of the weapon.
C.) Guns make things too easy
Excuse me but when your mage can reduce entire groups of people into charred remains this argument burns along with it. Oh but it's fine because only a limited number of people do it? That only makes it worse, only ensures the story even more revolves around that overpowered ability. Of course this misconception also stems from the poor understanding of firearms or the difference between historical and modern variants of it. You can't just "cap" somebody with a gun any better than you could perform that with other ranged weapons (or less so, matchlocks or even flintlocks aren't so responsive). As history proves dueling and melee combat remained an important aspect of warfare even in the 19th century. Your Smaug example is of course awful, there's no sense to it regardless how hard I'd try. So let me rephrase it. Does it really matter if Smaug was felled by a bow or a musket?
D.) Guns ruin that vanilla fantasy feel
Okay, news flash, Lord of the Rings actually featured gunpowder. Saruman used gunpowder to make a hole in Helm's Deep and Mordor's forces fired gunpowder incendiaries with catapults to burn down cities. I think you might have also heard about Morgoth's tanks or Numeorean ICBMs... although both were only found on scattered notes by Tolkien. Even without this revelation though there's a much more important message to deliver, be darned creative! If you wish to ape works of long-dead writers then that's your business, just don't expect to be applauded for it. Your entire concern of guns is based on misconceptions and an awful tradition. You have no logical reason to avoid firearms in a setting so it really just boils down to lack of creativity. And that's why I hate this behavior the most. If your only retort is that "but I don't wanna" then we're long past having a discussion about this.
Also let me clarify, while I hate this behavior with almost a passion it doesn't mean you must insert guns in every fantasy setting. If you run a campaign with a group then as the GM you have the right to determine what's allowed and what isn't. The problem is that there's that unreasonable revulsion to firearms to begin with. Lots of fake beliefs, bad traditions and often a lack of seeing the bigger picture. I am not forcing you to include guns in your campaign just because I want to. I am asking you to be open minded and realize the missed potential, the glaring double standards and the unreasonable fear concerning their inclusion to a setting. Also I am asking you to be a more consistent storyteller/writer/author or whatever. Lack of guns can be explained away if you do a good worldbuilding but guns themselves aren't the only thing you need excuses for. For example why your people have newspapers yet your environment resembles a town from the middle ages? Again, I encourage people to be creative.