With Mario, mind you that you're comparing a game that was sold at around the peak of the Wii's life cycle to a launch game. Completely different circumstances with one having an inherently larger user base than the other. Combining franchise fatigue (and we all know it's there) with a fresh starting console is going to result in less sales. We'll have to see though as Mario games do have long lasting legs so who knows.
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WarioWare Touched was a launch title (along with Smooth Moves) and they were the highest selling games in the WarioWare series in Japan. Melee was released merely two months after the GameCube's launch and ended up being the highest selling game for the console. A game releasing early in a system's lifespan does not mean it will sell poorly. To the contrary, if the game has long-lasting appeal the game will have legs to which the game will sell all throughout that game's life cycle.
However, you are correct about the phenomenon of "dropoff" from console to console. Layton, Final Fantasy, Phoenix Wright, Brain Age, Starfy and Nintendodogs all saw this phenomenon. However, the dropoff for Mario U and Nintendo Land (and the bombing of Game & Wario and DQHD) shows something more than dropoff at play.
The GameCube had an extremely low installbase in Japan (4.1 million was its lifetime sales there), yet software selling over 300K (and over 500K) was not uncommon. By comparison, the only Wii U title to have sold over 300K in Japan so far has been New Super Mario Bros. U.
The primary reason I brought attention to Game & Wario bombing is several things, all of which pertaining to Smash Bros. directly or indirectly:
1. It shows the Wii U is struggling A LOT. If the Wii U continues to perform as poorly as it does now, or only has a small recovery, there is a very good chance Iwata will no longer be the president at Nintendo by the end of 2014.
2. Iwata stepping down from Nintendo has implications on the future of Smash Bros. One is that Iwata is very close with Sakurai (Sakurai has worked under Iwata since he was a teenager), and it was Iwata's insistence that Sakurai even came back for Brawl (and possibly Smash 4 as well). With Iwata gone from Nintendo's leadership, there is a good chance that Sakurai will not come back for Smash 4 (or at the very least not direct it).
3. Iwata leaving or the Wii U have a rocky future means that Sakurai will not have the option to delay Smash 4 into 2015.
4. Sakurai may be pressured into trying to give more "pull" to the Wii U version of Smash 4 since that console is struggling far more than the 3DS (although the 3DS is definitely not doing exceptionally well in the West).
5. The continued poor performance of the Wii U will have long reaching consequences to the future of Nintendo as a company as a whole, and by extension the future of the Smash Bros. franchise.