So it might sound stupid and all, but there's a problem at Nintendo. 2D Mario players don't always transition well to 3D Mario players, as evidenced by sales.

For kids, perhaps full 3D titles like Sunshine, 64, Galaxy and the like are too complex to complete. Taking a kid used to 2D platforming and jumping right into an explorable 3D enviornment can be pretty daunting. I've seen plenty of kids just give up at Sunshine's water pumps and Galaxy gravity.  These kids never get to experience Mario games fully  and thus never complete the Nintendo indoctrination process.

2D Mario: Intro to Mario, the physics and basic lore of the games. Linear, single path to progress. Easy. 2D Marios are there to teach kids the basics. The tutorial of Mario gameplay. Level to Level progression. 1-1 to 1-2. Mutiplayer so others can help to progress (parents maybe)

Isometric 3D Mario:The new tier, gets kids used to the idea of playing Mario in a 3D enviornment, not too hard to understand where to go next.Isometric nature of the stages helps to show where to go next. Possibly grouped progression (More freedom to choose order of levels) or Level to Level. Also has other helping to a degree to progress.

"True" 3D Mario:Fully 3D Mario with exploration elements. Not as linear as the other two, branching paths. Full freedom of 3D movement. "Harder" than the other two and usually the deepest in terms of gameplay and level design. Usually single player experience (aka need no others to enjoy fully) at the stage where a grown person can truly appreciate it.

/end rant.

2D Mario=the best tech of the time
3D Mario=the evolution of Mario
NSMB=throwback to a style gamers wanted
3DLand/3DWorld=2D Mario w/ 3DS showcase

That's how I see it. 2D Mario is there to make money from old farts like me and little kids. 3D Mario is there to showcase the power of the hardware.

I think you nailed it. There are 2D Mario games, 2D 3D Mario games, and Full 3D Mario games. 3D World/Land are simply 3 dimensional versions of the 2D games' style of playing.

Yes, as evidence by the fact that the 2D Mario games generally sell alot more than the 3D ones.  The 2D games appeal to both younger gamers as well as parents / old fart gamers that grew up during the NES days but haven't really gamed much since.

The 3D games are geared more towards the "core" gamers / Nintendo fans, as they are more complex from a gameplay standpoint.