Circumstantial evidence. If 2D platformers are easier to develop, have lower costs, lower risks, and higher Return On Investment, then why aren't major 3rd party studios cranking them out as their main releases? Particularly in the Wii/DS era when the NSMB games sold over 30M a piece? When you point to sales, you're referring to a small handful of AAA 2D platformers released in a sea of FPS', cinematic action games, and licensed sports games. A couple of isolated data points. Clearly Activision, Capcom, and EA didn't look at NSMB Wii and think "there's our gold mine, forget Assassin's Creed and Madden, let's make our NSMB killer!"
That's the disconnect. Sure, Nintendo made a small handful of enormously successful 2D platformers with their legendary IP, but most publishers didn't think there was a bigger market there. There's a commercial perception that platformers = cartoony = kiddy and most major publishers today specialize in games with more mature themes. And until someone like Ubisoft challenges that perception with a AAA retail mature-themed platformer, then that perception will hold. For the record I think that they absolutely should go for it and try something like that. In the right hands that would be very successful.