The thing is nobody will know anything about some Justin Timberlake in 20 years, while everyone will know who that Michael Jackson was. Master Chief is just a weak character, the only thing he has going for him is his
badass name and
badass attitude. Other than that he is just your average soldier. Halo as a franchise is a different story.
We can begin to compare Master Chief and Mario when you'll see games selling only because they are named after the Chief or you see virtually everything in stores with his face on it. Pretty much every Mario merchandise possible exists. There is Mario soap, Mario food, Mario clothes, Mario everything. The Chief is not very marketable outside of his very limited Halo world. He is totally unknown to the general public, Mario was even a question on Who wants to be a Millionaire here!
The time of Mario being more popular than others is fading away, but i don't think the Chief is going to replace him. Really you have to think on a broader scale, not just in your limited video games world. Those first day Halo2 sales really got to some people heads