Lil B can "sell out a show" doesn't mean he is mainstream. And stop acting like the average person in the street knows or gives a fukk about classical music. It's for a niche audience. Just like street rap. Hip Hop more than any other genre is about the youth and moving forward. Hip Hop does not respect it's elders or things that came before it. What gives you the idea that their would ever be some mass movement in hip hop that references the past. For every Joey Badass in NY their are a dozen French Montana's.
If he can sell out a show who gives a fukk if he's mainstream? Dudes like you want half the money, and all the hype.
I'll never understand why people like you are so obsessed with "the average person". Them composers selling out shows every night do not give a fukk about how "pop" from The Coli feels when they're cashing their checks, or how "the average person on the street" feels about classical music.
A good example is Freddie Gibbs. Dude dropped one of the most critically acclaimed albums this year, any genre, and is currently on a nationwide tour. Completely independent. And he's a gangster rapper. You think he's like

"will gangster/street rappers ever have mainstream appeal?" when he's counting his money?
Fred.