You clearly don't know what Mario games are about if you think they're about "rescuing the princess".
Agreed, Mario is about level design and tight controls. If anyone knows of a better plat former released in 2012 than NSMB2 let me know because that was a joy to run through multiple times.
unlike many other games, the learning curve for Mario games is almost non-existing, and not because the games are easy, but because the controls are so damn goodlike!
And if Nintendo rehashes so much and still sell boat loads, why hadn't anyone else make a competent platformer to dethrone Mario?
Because Mario has been consistenly building a fanbase since the NES days. He's been involved in several of the most highly acclaimed games of all time. He helped define console gaming with Super Mario Bros, and 3D gaming with Super Mario 64. Few series can match the consistend continued excellence that Mario has maintained for over 25 years.
It's simple why nobody has made a platformer to dethrone Mario. Mario has a well deserved reputation. Gamers of all ages and experience levels know if they pick up a Mario game they're going to get an incredibly fun experience, and aside from a few missteps, they've always been right.
There have been other very good platformers. Little Big Planet, Donkey Kong Country Returns, and Rayman Origins, were all great games. However, none of those games were part of a franchise that has been so good for so long and has done so much in shaping gaming in general. If any franchise is ever going to topple Mario they're going to have to work for at least a decade at building a fanbase, and they're going to have to put out some truly revolutionary games to gain attention.