I swear if OP was born in the mid 80s I'm going to bury my head in the sand somewhere because I'm embarrassed to be a part of that age demo at this point.
@Ziggiy already said it best with science and facts but how in the hell are we having a discussion where a rapper with several platinum or multi platinum plaques, a program in Harvard named after him, a documentary on his first album touring the country as we speak, and several grammy nods NOT mainstream? What is proper mainstream? That's vague as fukk and an extra Mr. Fantastic-style reach to discredit the brother. He had his peak in the mainstream, he moved on to another phase of his career and his music has been better in the 2nd half than the 1st. I get a lot of you cats don't like Nas for some reason, cool. I get a lot of you are kids (and I mean that literally) or some of you are still holding on to a 13 year old grudge that both Jay and Nas have let go because they're grown ass men, but give the dude his props.
You can be a fan of both (I am) and still be critical of the music and moves each have made but damn this shyt has to be a parody at this point because any subjective person has to say that the dude has had a pretty great career and he's gotten plenty of mainstream love. Someone silly said that his albums sold that many because of his diehard fans....

so all those diehard fans that didn't buy illmatic because they bootlegged it, all of a sudden got a change of heart and went out to get It Was Written? Oh ok I got you. He became a crossover artist which is what a lot of his diehard fans crucified him for, get over the shyt