They say the sport would be in trouble if he leaves due to sponsorship woes. I can't really see how Rossi himself brings so much money into MotoGP. I mean I can't see sponsors like Repsol, Fortuna or Malboro leaving if Rossi pulls the plug. At the very worst Nastro Azzuro will go with their favourite son (But how much money do they contribute anyway?) and Camel might join Gauloses on Yamaha's .... list of tobacco companies. But even if that were to happen, which I think is unlikely (Camel have got as much chance of exposure on the podium with Edwards and whoever takes Rossi's seat as they did with Biaggi)
We're talking about a factory team, not privateer or a satellite team. If push comes to shove Yamaha could fund the team themselves, like Suzuki and Kawasaki.
I understand that Rossi creates attention which is good for all sponsors, but I don't think his winning streaks and antics draw that much attention to the sport from the outside, and even to people who follow MotoGP, we don't sit there and go "wow, Rossi's wearing a pirate suit, But there's a michelin blimp!!! I know what tyres I'm going to buy now!"
I think Rossi's move to Yamaha will have a lasting impression in MotoGP, even if he leaves. His presence has helped Yamaha take motoGP a lot more seriously, and as a result the M1 now is a lot more competitive than the first generation bike, and it's not just Rossi. Edwards came in 4th, remember? So the factory M1 is really on par with the RCV now, which is good. It's good to see more competition coming from the manufacturers. Although a technology "arms war" could take it's toll on smaller budget manufacturers.
That said, Ducati is a fine example of what innnovation can do in place of investment. Easily the 3rd best bike on the grid now coming from, financially, the weakest factory.