Tom that was a good way to explain it. Only thing I could add is that for a lot of champions the legacy comes from winning against the odds. Enter Mick Doohan, the image thats burnt into my memory is seeing him hobble out of hospital for the last race in 92 to try and win the title. Even Rainey, no ..... himself said he was amazed to see Doohan trying to ride in that condition. And he nearly did it. Then Doohan went on to 5 titles with said leg and fused ankle? So Doohan overcame what many thought impossible. Rossi has hit the first major obstacle that I can remember in his whole career and I bet he'd love to be able to write his biography with a happy ending. Winning on the Ducati removes all doubt.
I hear what you are saying, but nobody (not even Valentino Rossi) can win forever. In the old days it wasn't uncommon for riders to be forced away from racing by injuries. Thankfully that is considerably more rare now, but when that did happen people generally tend to extrapolate those careers at the highest level for the considerable future (the most extreme example is senna). But anyway, nobody can continue winning forever, and at least with Ducati his legacy will be one of a challenge too far, rather than him staying at Yamaha to be beaten by Lorenzo.