Honestly, I don't think platformers have faced any kind of decline. While Mario may be the best of them, that says more about the quality of the franchise than the state of the genre. There are still plenty of other games out there for platforming fans to enjoy like Sonic, Donkey Kong, Kirby, Rayman, Little Big Planet, and a bunch of other gems that don't get the attention they deserve. It may not be the most advertised or popularized genre in the industry anymore, but it's still a big one. Aside from the typical colorful and charming games we've come to expect from the genre, platforming has played quite a major role in plenty of other popular AAA games like Uncharted, Tomb Raider, and Assassin's Creed. While platforming isn't the only focus of either games, they definitely prove great examples of how the genre has expanded over the years.