By your definition, neither graphic adventures nor platformers are niche either because a good number of people buy Telltale's graphic adventures and Nintendo's platformers. Nintendo systems aren't doing that well, but certainly, Mario is still selling pretty well despite that. In addition, puzzle games aren't niche... They just moved to mobile and became Free with IAPs. The way I see it the Souls games are in this weird state where they certainly would be niche if compared to the games people usually hype up, but unlike say Platinum's games, they end up actually selling to a decent amount of people.
Uhm, not really good examples I think. First let me state again that I don't believe sales have a lot to do with a title being niche.
Said that, while it's true that Nintendo's platform games sell well, we should see the whole picture and wonder: what about the others? What about all the platforms we had 10+ years ago? I don't remember the recent Rayman games selling very well and that's even a known IP. Sonic? Nintendo's platform games sell and they don't leave Mario and friends for the same reason they put them in party games or golf games or whatever: they would appeal and sell way less than they do because the gamers asking for them aren't a lot.
The market for platforms on console shrunk a lot, they survive pretty much thanks to low budget titles and / or indies.
Telltale's graphic adventures became known thanks to the license of The Walking Dead, now also Game of Thrones. I don't know the sales but I'm ready to bet they both did a lot better than The Wolf Among Us or Life is Strange, definitely more than D4. Is it confirmed that they're working with Minecraft now? Well, another totally unknown IP, isn't it. Keep in mind we're also talking about cheap episodes sold on almost every system available, even mobile. The graphic adventures we were used to are now gone / rare: Policenauts, Broken Sword, Syberia, Monkey Island.. no episodes or licenses, full price, a model that's basically impossible today, I'm afraid.
Point is, if they weren't niche, we would have a lot more titles like them, even riskier projects, but it's not happening.
Mobile market is different, people buy what's cheap mostly and games like Flappy Bird can sell like crazy. There's nothing really niche there in my opinion and this is why puzzle games can find new life there with good games like World of Goo. It's not Braid nor Portal and the only way they can be sold is for very, very few bucks or for free, but it's something.
To make it short: to survive doesn't mean not to be niche, but I'm aware that my idea of niche is pretty uncommon or unshared.
