The DS had Mario 64 DS as well, and that was a launch game.
While I do agree that the release of NSMB2 at this particular time could curtail sales of 3D Land, I don't see the problem with one of the sidescrollers per system--one on the DS, one on the Wii, one on 3DS, and one on Wii U. With the NES, there was actually a year where we got two of them on the same console. Of course, I do think they should worry about brand saturation with all the unrelated games that get his name slapped on them.