Pornhub, I think, is a poor example. Blue, red, purple, vegan, carnivore, white supremacist, antifa, cops, firefighters, priests, atheists- all watch porn. It reminds me of something I heard said about Howard stern years ago. The average Howard stern listener- the story went- listened to him for an hour (?) a day. The average Howard stern haired listened to him more than twice as long every day. And, porn is hardly a niche market.
How do you make a small fortune? Start with a large fortune and open a restaurant.
As to competition, the question is not “what advertisers would they be competing for?” The question is “what advertizers are left?” Can you attract enough advertizers to make a video hosting site viable with a gun only focus? I don’tknow, but I’d wager you better start with deep pockets. Like opening a restaurant, it sounds like a great way to make a small fortune, out of a large fortune.
An increased breadth of audience increases the pool of potential advertizers. As an analogy, it was stated recently that gun buyers also buy soccer balls- a lesson that Dicks is learning the hard way...