Useful for what?
You address a couple of things, so I'll try to cover them in the order your post does.
Firearms and concealed carry don't really have anything to do with hand to hand. You aren't going to do much training in martial arts that specifically addresses firearms just because it's not necessary. If you're close enough to engage someone with a firearm, you'd use the same methodology to attempt to negate the firearm as you would any weapon of a similar size. If you aren't inside ten feet and a gun is already pointed at you, you're fucked. If you're inside about 20 feet, and the firearm isn't drawn, they're fucked if you can apply any control to it at all because they won't get drawn and fire in the time you can close distance as long as you're in decent shape.
Doesn't matter if it's concealed or open carry tbh. If anything, a gun is easier to control than a knife, but that's a tangent that's not applicable here.
For me, and I've been shooting since I was maybe elevenish, I'm still not going to draw, remove safety and shoot fast enough to ensure a stop on someone inside of about twenty feet if they're already primed to move. You might get the fast draw trick shooters that could, but they won't be doing it from concealed carry.
Besides, you see a weapon of any kind, number one goal is escape, not fighting. The only reason you'd engage with a firearm user is if you can't escape. Same with a knife, a stick, whatever. Fighting isn't the goal, you don't want to be fucking around trying to "win". You do the bare minimum to gtfo.
Seriously, it's not a factor in the practicality of martial arts.
Where martial arts is useful for the average person that's maybe gong to use the training in self defense once or twice in their entire life is in being prepared for trouble. You train, and it's good exercise. You develop a sense of how your body works in motion related to another person. You learn how to react to pressure (with a caveat I'll cover in a bit). You learn how to take hits, how to judge distance and how to close distance. And that is true for any training that isn't just katas, even systems like aikido or judo that aren't meant to be self defense as a primary focus.
Now, the caveat to that is sparring. If you never, ever do any training with a partner that's resisting your effort, it's just fun exercise. That's where aikido usually fails, the near total lack of actual resistance while training with a partner. But the basic techniques if you do resistant training and toss the stuff that doesn't work are literally bone breaking even with aikido, and it's as gentle as it gets.
The problem with kung fu, karate, or any traditional martial art is the training not including live, resistant sparring. Even systems like mma that's had the ineffective stuff removed, if you don't train against someone that's working against you in a realistic way, it's just fun exercise. But there are "styles" of pretty much all of the name checked systems that feature live sparring.
But, in real world scenarios, if you do that training, if you spend the time repeating a technique against a resistive partner, you won't have to try and use it. You'll just react. And that's how martial arts are useful in self defense scenarios. Instead of having to see an attack, decide what to do against it, then make the attempt, you detect the incoming attack, and you're responding without any conscious decision. You basically taught yourself a trick the same way Pavlov taught the dogs to drool to a bell. That's a gross oversimplification, but it's good enough for this.
Now, it takes time to reach that automatical response. That time also takes money most of the time (unless you know someone willing to train you for free, and good luck with that). So whether or not you want to invest that time solely for the chance you might need it, that's another tangent.
But, if you do choose to train, that's why it helps to get something other than just the fighting out of it. The fitness, the fun, the camaraderie, the self discipline, the self awareness, the pain tolerance, there's so much you can get out of it, if you're willing to put in the resources.
Now, the post references specific "arts". But it doesn't have to be traditional arts at all, or even eastern traditional arts. You can get all the same benefits from boxing, wrestling, HEMA, or any of the arts that developed outside of asia. And we've got mma now that focuses on full contact fighting, and has whittled away at the stuff that's not effective for full contact sport fighting, which makes it pretty damn good for self defense overall.
Again, guns just aren't a relevant factor in choosing to learn martial arts or not. Even melee weapons aren't. The primary advice you get, even when training to counter weapons, is to not let yourself get into that fight in the first place. You run first, you try to deescalate, you keep situational awareness to hopefully never even need to run, any of the things that could avoid being close enough to the weapon to have to control it at all.
But, all of that is helped by training. Situational awareness itself takes time to develop.
And, yes, I'm kinda enthusiastic about the subject lol. But, as much as I love/loved martial arts, it isn't for everyone. It isn't necessary for daily life for the average person. It's like an insurance policy where you pay in now, in the hope that if something happens later, you'll have it covered.
As an example, me and my best friend are the same age. He has never been in a fight as an adult, never been mugged, attacked, or even threatened with something like that. Me? I can't actually remember how many fights I've been in, but I was a bouncer for a little over a decade, and worked some really bad areas as a nurse's assistant. If I hadn't been doing those jobs, and I discount any violence because of them, I would only have maybe a dozen fights have happened.
Is that range of self defense occurrence worth the resources? It was for me, but it might not be for someone else.