One thing that wasn't mentioned is that there is a rider that has won in the vicious 500 days, the ferociously crude 990cc bikes, the more refined 990 bikes, and the digital 800's. One guy has been able to beat the best at each kind of bike.
I am confident that had Rossi been born 25 years earlier, you would have seen him sliding a skinny tire Goodyear-shod Yamaha or Suzuki around the tracks at or near the front. Or if he rode in the late 80's he would have been stuffing an NSR into Laguna's Turn 11 and fighting the bucking bronco highside on the exit to take the checkers.
I think KRSR is selling himself a bit short, the great riders adapt to the current conditions. Roberts might not have had the same style he used to win, but he would have quickly developed the skills to be the fastest around a track.
This is why in the past I have said that the really "good ones" make an impact almost immediately when they go to GP. Rainey did, as did Schwantz. Doohan and Lawson also. Today, Stoner/Pedrosa/Lorenzo have shown winning speed in just a few races of experience. Other riders get into the series and seem to constantly be stuck in seventh place, never to improve.