How many other groups from that era are on the level of Run DMC or Whodini? Album/music wise I don't think Furious 5 had anything competing with them, same with Treacherous Three, I can't even think of many other groups that had albums out between 80-85.
The next era would have to start at 85 at the latest wouldn't it? 85/86 is when you see BDP, Ultramagnetics, etc and they are decidedly new era compared to Run DMC. I'm speaking pretty much on wax and as I said before I'm not well versed in pre album Hip Hop as far as Cold Crush etc and I would personally have them in their own category because wax is a different thing.