I dont think there is a correlation between hip hop music sales and rappers out to make money staying or quiting. People don't just want to be rappers to make money, a lot of people idolize the 'lifestyle' a rapper lives, the money, the women, the drugs, the traveling. Rappers are no longer just gangbangers, they are rockstars, and everyone wants to be a rockstar, so expect a lot more of the same.
What you will see is less and less major artists. Mainstream Hip Hop is in dissaray and companies aren't out to sign big contracts because the content just doesn't sell 7 million records anymore. There is no reason in any music genre to sign the next big flop.
Now the positive side of this development is that rappers on a midsized stage like Keak da Sneak or Hieroglyphics will start to draw larger audiences because the walls that limited radio play will fall and fans will start saying "hey that dudes from Oakland, he's not on a major and he's selling 50K, there must be something tight about him" and picking up on a new artist.
But sadly, I think the over saturation and the bullshit artists in it for the money will always be around.