This point doesn't really work as the star coins are taken from 2d Mario. They're from the NSMB games. I also disagree that Nintendo has any intention of trying to get people to play 3d instead of 2d. For a start 2d Mario is much cheaper and quicker to make. They'd rather 2d sell more than 3d.
You don't find star coins in 3D world. You find stars. The experience of finding stars in 3D World is just like finding stars/suns/moons in the other 3D Mario games. 3D World starts out as "flagpole Mario" and gradually transitions to "starfinder Mario".