The old school Melle Mel approach - Melle Mel was the best at it, but he wasn't the first one to rock a style like that. I'll argue that his style grew out of what was jumping off at the park and community centers. Where those cats got it, seems like a "natural" transition from Herc's one liners, the call and response, to emphasizing the tail end of the call and response.
King of Rock, there is no HIGHER
Sucker mc's, call me SIRE.
or
Put your left leg down, your right leg UP
tilt your head back, and finish the CUP
Now my argument, without going back and listening to every record and recording the dates and checking the playlist coverage, is that Rakim was the first to really flip a style, that didn't end with mad stress on the last word of the line. This happened 13 years into the game, so it was real discovery./breakthrough
1) his style opens up lines for more words and more syllables than previously thought
2) his lyrical content was just on some other shit. He was doing something completely different than just dissing sucker mc's and talking about his day - that's a big conceptual jump to me.
3) Most importantly, he showed the rap world that you could be ridiculously dope with a new and different style. So even if you didn't copy Rakim, you no longer had to sound like Melle Mel. And when you listen to records from those days prior to Rakim, they all did the same sort of last word stressed old school rhyming. So after Rakim, you could come with a totally different style than Rakim, where before you had to sound like everyone else. - This is an even bigger conceptual jump.
That's my argument. I think to prove it, we'd just need to get a list of dope records before rakim, and just see if anyone was really doing something really different than the old school style. You can pretty much put all those names on a timeline, and maybe on a map. Then do the same thing for the mc's after Rakim and see what happened.
Indeed, the question you should really ask, of the NYC cats who dropped records before and after Rakim, who stuck with the old school style?