Rossi jumped ship from the all conquering Honda to a manufacturer who had done nothing for a decade and ended up with the world title in his first year. Marquez jumped ship from Honda to the dominant manufacturer. Big difference.
Rossi was never on a shitter, apart from the Ducati that he couldn’t get the hang of. But he soon jumped ship, fair enough, only Stoner had joy with the Ducati back then, albeit the 800. Rossi’s mistake was making a powerful enemy of Marquez. It didn’t end well for him (Rossi).