The Genesis launched in 1989. It was the number one 16bit console until late 1994. By 1995, the Satarn released so it's pretty safe to say the Genesis beat the SNES.
Sega Genesis sold about 40 million sold and SNES sold about 49 million. How did the Genesis beat the SNES? Are you arguing that within a certain time span the Genesis outsold the SNES?
The Genesis sold about 30 million units worldwide and it beat the SNES in EMEAA (9 vs. 8m). I think the SNES overtook the Genesis in America before the Saturn launched and in Japan the Genesis got totally crushed. That's where most of the 19m gap in worldwide sales comes from.
Anyway, on topic: NSMB was #9 on NPD's multiformat charts in June. This marked the 62nd month for the game. So despite the apparent threat of iOS and smartphone gaming, Nintendo's biggest IP was still going strong. Mind you, not a new release, but one that was already more than five years old at that point. There's a simple recipe for Nintendo's success and it's solely up to them to execute it.
Basically, there are two ways Nintendo can be taken down:
1) Super Mario Bros. will be made obsolete by a competitor. After 25 years it's pretty clear that this isn't even an option.
2) Nintendo destroys themselves. Now that is actually feasible and exactly what they are doing right now.