"The hip-hop game is about staying and sustaining and keeping it going," he said. "You’re a great rapper, but they can’t just put you in a rapper box, because your music is going in so many different directions. You’re a rock star, and it always was like that. It comes from the hood. It comes from the mud and then it blows up, and then who’s going to hang around? Who’s going to be the one that sticks around and keeps giving us that excitement? That’s what makes you great, when you can maintain in this game."
Nas is absolutely right. Greatness is not a single moment, but a career-long goal to be achieved. Here, he commends Scott and his "sound" for being so hip-hop, yet so outside the box of what we conceive to be hip-hop. The best way to sustain a career in the game, then, is to continuously grow and push yourself. See also, Kanye West.
If you are not constantly trying to one-up yourself, then how on earth can you last in such a saturated field? The grand lesson of Nas' comment is twofold: think about your legacy when you make music, and never stop growing as an artist. That's how you last in hip-hop and avoid being a one-year wonder.