Yes no platformer of similar quality will sell as much as Mario. No question about that. But has any fitness game sold anything close to Wii Fit. The biggest selling fitness game that isn't Wii Fit is at around 3 million. Wii Fit is gone past 22 million. Why did EA not decide not to release their Fitness game when they knew it had no chance to sell even a 10th as much as Wii Fit? Obviously because making a fitness game is easy. Why have third parties decided not to make a quality platform game knowing it won't sell anything close to Mario Galaxy but will at least sell like 2 million if it is of a similar quality? Because they do not have the talent to make such a game. Why did no one make a quality FPS from after Red Steel till COD knowing there would be no competition on the Wii but instead they were releasing Guided FPS? Because making a guided FPS is easy compared to a proper RE style game of FPS.
Truth of the matter is they all missed a huge opportunity. Especially last Christmas period when all there was from Nintendo was a Wii Music game no sane person cared about and Animal Crossing which was a port of a DS game. If any third party had released an exclusive quality Wii game last Christmas they would have made a sales killing. Instead their lack of talent and foresight meant they decided all Wii owners want are rail shooters and party games.
You just don't get it.
I know it is pretty pointless to argue about Mario games but I bet there are many companies which could make a platformer as high quality as SMG is. It would cost relatively high amount of money and when there aren't many examples of a platformer doing so well that it would justify that kind of budget. During last generation only one platformer sold more than 5 million and guess what? It was a mario game.
Crash bandicoot sold a lot on ps2 but it did not have as good competition as a platformer would have on wii.
The point is that many dev teams can make a high quality game on wii, they just see the risks being too high to make the game. It is not like they can't make a high quality game, no matter how much you would like to believe in that.
Crash Bandicoot: The Wrath of Cortex sold over 6 Million copies on the PS2 and XBox combined, and several Sonic games had (roughly) 5 Million unit sales when you combine sales across platforms ... This is even though most games released using these characters have been awful for several generations. Jak and Dexter, Ratchet and Clank, and several other platformers have had very respectable sales.
Even very shitty platformers generally sell over 500,000 copies on a single platform, and regularly pass $1 Million when they're multiplatform, and the risk of the genre is pretty minimal; and yet there is no real effort put into most platformers outside of a couple of first party games.
The reason why there are so few quality western games that are not shooters is very similar to the reason why Hollywood doesn't produce many decent drama movies anymore. The industry has moved away from making reliable profits producing a decent product at a moderate cost, and will blindly seek out massive revenues at high risk ... For the cost of a HD game most companies could produce several Wii games at varying budgets (from low budget to high budget for a Wii game) across multiple genres and produce reliable profits, but they favour to put all their resources in one basket aiming for a big success; and more often then not, they don't get the big hit they're looking for (and end up with record losses along with record revenues).