Well although I'm not sure about New Super Mario Bros Wii selling 20 million copies but we all agree it'll be quite successful.
I think people are taking the 2d and 3d thing just a little too much over the top. I really think the only place it might matter is maybe Japan and I doubt it's such a huge problem that the regular Mario fans wouldn't buy it.
I mean we have seen that the 2d mario games have sold better but maybe we have yet to think of the circumstances. What in any context on any platform could compete with Mario back in the 80s and early 90s... nothing really. Thus why even the non bundled Mario games sold incredible. We move up to the more recent ones of N64, GC, and Wii and we see that they've had a lot more to compete with on an industry standard and of course a Nintendo standard as well with the rise of things like Mario Kart, Zelda, Pokemon, etc.
Another factor for the success of the most recent 2d styled Mario game on the DS, is that it was essentially the big DS game for a long time. Still is the essential DS game. Actually there is a really good comparison for it... Super Mario 64 on the N64. Sold to a 3rd of the userbase.
So really I'm not sure it is so much to do with the dimension the game is presented in because that would very much so be logically proved wrong by the case of Super Mario 64. Now Super Mario Galaxy sold a solid 8 million and New Super Mario Bros Wii will probably sell a decent amount over it. But that in no way implies the death. Hell you could even argue that with a franchise like Mario that due to the expansion of the userbase so greatly since 2007 that Galaxy 2 and NSMB Wii will actually be benefited by that to help increase sales.
So as I said the 2d/3d thing may have something to do with (moreso in Japan) but not as large as we are making it out to be. Even more so the reasons I stated, that it's a remake of the first Super Mario game which is still seen as the biggest game in video game history, and that even difference in sales 8 million is quite a bit.
