To be a legend someone should have done something legendary. Achieving popularity and commercial success is common ground, any mainstream artist get to this point. Now, doing something remarkable that stand the test of time is more close to legendary, like when a new paradigm is created. Think of bands/singers that fueled/started new trends, the pionners, etc. The real legend actually will be credited to great things even when isn't really deserved, the way story develops and how the artist build the 'fantastic' aura that surrounds it's work and life is important.
