2D outselling 3D in any form has never been front page news.
Just pointing out that it has been to me, although basically only in Mario, since that's one of the few instances I know of. I don't know what Donkey Kong Country Returns' sales look like compared to the 3D entries though. It's possible they're better too. The Wii has a lot of 2D games to appeal to the old school hardcore fans, if they can stomach the fact that the themes and styles are still mostly still aimed at young kids.
What isn't news is that 2D Castlevania is widely considered to be vastly superior to 3D Castlevania.
It is among the old school hardcore fans, but your "widely" changes that by bringing larger segments of the population into play, so I'll go with that. What is generally referred to as "hardcore" in gaming marketing today -- the "core," the segments of gaming who are represented largely by the PS3 and 360 -- will tend to prefer a 3D Castlevania. These are the people CV64, LoD, LoI, CoD, and LoS were intended to reach, as well as those among the old school hardcore fans who would like a 3D action CV too (and it obviously can still appeal to that group as well).
But then you have the new swath of "casual" gamers brought in by the Wii and who trickled in for earlier consoles (perhaps some had a PS2 as well but probably few of what those of us here would consider great titles). These are the group that NSMBWii most appeals to and where a great deal of its sales come from. And it appeals to the old school hardcore fan too due to nostalgia and being familiar with the style from younger days. So you essentially have concentric rings of larger and larger groups. NSMBWii largely skips the core and appeals to the old school and the casual.
Now when we consider what is considered "widely" better than something else I assume we're including all these groups. The thing is, this new casual group doesn't really "consider" CV at all. They probably only vaguely know it exists if that. So then we get into the other two groups, the old school and the core, and there we see the core's preference for 3D trump 2D CVs. So that is essentially what makes 3D CV "widely" trump 2D CV. The thing is, the majority of the casual group probably wouldn't like 2D CV even if it knew it existed, for other reasons -- mostly stylistic and theme related, although because of the casual group's large size, it's possible for even a minority of them to trump the core in size and sales.
Keep in mind that as with any demographic analysis, these aren't hard and fast groups though, there's some level of abstraction and bleed over. It's perfectly possible for someone who played the 2D CVs as a kid or young adult to have grown up to prefer the 3D entries completely or still have fervent desire for both 2D and 3D entries, and you'll see these groups represented here as well.
So if Konami designed a 2D Castlevania with the same pride and diligence Nintendo showered on 2D Mario --what do you think the sales would look like?
Probably a bit above DS CV level sales, assuming it's on a platform with a good install base. Pretty modest cult hit that's very acclaimed by professional reviewers (although the DS CVs already are pretty liked by pro reviewers actually). Now if by pride and diligence you mean in large amounts of actual marketing which Konami simply doesn't do for CV outside the game mags and sites and such (which doesn't reach outside segments of the old school market and some of the core market, if that), then I'm not sure. That muddies the waters a bit and it might be able to sell higher. Mind you, still nowhere near NSMBWii or some other Nintendo first party stuff. I doubt it'd hit 1 million, but it'd probably be better than DS CV levels.
If the NSMBWii example existed before Konami started the Dracula X Chronicles, I'll bet IGA could have pushed for an HD console release.
I have to disagree sadly. Castlevania and Mario are just in totally different universes, so Konami seeing what NSMBWii can do largely would have no impact on their CV strategy (what they'd allow IGA to do). What works for Mario does not work for Castlevania, not only because Castlevania doesn't have the marketing Mario has had, but also because NSMBWii is a somewhat different type of platformer from CV (even Rondo) and has a vastly different style. Probably a large percentage of NSMBWii sales are due to the steady Mario marketing, and longing for a simple Mario platforming experience like the old NES-SNES styles before all this complexity came into gaming, or someone new to gaming (the casual wave) who also wants something simple like that.