There are many good points made here. I have seen some declines, but honestly, when I worked in a retail gun shop, I had more cheap used in-lines than I ever wanted to see. My boss hated the side-hammer guns, so when I went up to his store, I grabbed all I could get for my store. Flintlocks were rare, but I sold them all within a couple of weeks, be they T/C, CVA or something else. A rare custom gun would go quickly, though I never really saw the 2K plus guns.
Now there was a time that I lived through where you could not give away a flintlock or a side-hammer caplock. Only the Knight and later on the T/C, CVA and higher end Austin & Halleck guns would sell without major effort. Even during those times though, I could still sell the traditional guns.
I also remember a time when nobody wanted AR platform guns... those suckers, made only by Colt in those days... were expensive at $300.00! Times changed.
There are still people using vintage cameras and making Daguerreotypes just like Matthew Brady did during the Civil War era. Others use Leica screw-mount cameras, vintage Nikons and even Minox spy cameras.... and there are still people who process film in the digital age.
There are more blacksmiths around now than there were in 1950. It made a resurgence right about the time that most smiths were retired or dead thanks largely to a man named Alex Bealer who wrote
The Art of Blacksmithing before all the smiths died out. I am proud to say I was one of his "students" in that I bought his book and assembled a smithy.
The point that I am trying to make (albeit doing a poor job) is that nothing ever really goes away permanently. There will always be people shooting muzzleloaders and people who make and service them. Some time after Elon Musk gets his Mars colony started, eventually somebody will build and shoot a flintlock on another planet. I will likely never live to see that day, but I sincerely hope there is one day a reenactment on Mars... even if the participants have to wear an environment suit.
Yeah, I know I'm crazy, and they're comin' to take me a way hee hee hoo hoo to the funny farm...