IMO, if quality over time were graphed, it would be a bell curve.
The 80's were good (for the time), the 90's were the best and the 2000's were ehh (because we were spoiled by 90's music).
The standards were low in the 80's. Anyone who could do multi's in the 80's towered above the rest. If you took some of the average-to-good acts of this decade and put them in the 80's, they would be great too. if you took busta rhymes and you put him in 1985, he mightve been one of the all time greats easily.
i'll even make a really controversial claim: even rick ross in 1985 would have been considered a great rapper. but definitely not now.
rakim, run dmc, nwa, etc. profoundly influenced a lot, but their rhyming wasn't above what we had in the 90's.
well you could say without them there would be no jay z or nas. but i could also say that without Chuck Berry, there probably wouldn't have been rap either.
actually, we're even seeing today, that a lot of the old "Greats" are coming up short today because they haven't changed much compared to the constantly changing standards.
i think now in the 2000's most of what worked in the 80's and 90's has gotten stale. and there's a big need for innovation that just hasn't come. someone who figures out what hiphop is missing (like Eminem in the early 2000's) could do very well.