Excellent insight from Lindy. The heyday of the 3D platformer is indeed over. So are a lot of other genres too. The problem is, new genres aren't exactly filling in the gap. We have the sandbox genre from last gen thanks to GTA, The music game genre which has been ran into the ground in the space of four years, and Resident Evil 4 updated the third person shooter genre.
As a trade off, JRPGs, Strategy games. Flight sims, 3D Platformers, 3D fighting games, Survival Horror, Strategy RPGs, and even racing games to an exent are all faded/fading away.
Soon we will be down to FPS and Third person shooters with the occasional Third person action game thrown in.
This is why Nintendo went blue ocean and created Wii Sports and Wii Fit outside the stagnant waters.