The sport existed before Valentino Rossi, and it will exist after Valentino Rossi.
Some may well say that he heightened awareness which to a degree may be true, but these same people overlook that he happened to be right place, right time with right talent and a seemingly engaging personality. He rode the social media wave, the stardom and fandom like nobody before and certainly he did change the way riders presented themselves, the way they used the various mediums to further themselves and their earning potential.
Many will say that he bought sponsors to the sport and while that has a ring of truth to it, many of these sponsors were bought by the coverage that the sport was receiving due to Rossi's smarts in his use of social media. Yes many associated themselves with him, but many moved on from him to other riders, teams or manufacturers so one could well say that he played a part in these sponsors arrival but is not the reason they remain
Along with the above, Rossi was a master manipulator. He played the media that covered the sport with the aplomb of an intensely skilled concert violinist and had them eating from his hand and publishing articles in a way that he led them to publish such was his charisma. He became the face of the sport because the media went to him (or were shielded to him by DORNA at times) which raised his seeming importance to a level the sport had not seen previously.
All of this while maintaining a racing skillset that allowed him to remain competitive and win many races as well as titles while also allowing him to build a business empire not seen from athletes in the sport previously.
He was MotoGP's Tiger Woods, and like Tiger things fade and influence or relevance diminish for any number of reason. In Valentino's case it was centred around opposition as we saw an influx of talented young riders who were no longer in awe of him. We saw riders that know how to play the ever smiling, happy go lucky, always up for a chat type of riders while at the same time have a mentality that would see them smiling at your face while kniving you from the back. This is a business and they know how to play it and when the numbers capable of this grow then you have lost the influence you had in the past.
Today, people know of Rossi, but the same people KNOW Marquez, Quartararo and the likes.
Valentino's time was yesterday and the sport has moved on and while it will never forget Valentino, nor should it as he is a part of the sports great history, he will be just that, history as we move on to the next rider or riders that the world falls for.
None of this is to diminish the achievements of Valentino as no matter what people believe, feel or which conspiracy theory we hope is true, facts are he has the trophies and titles to prove he was a somebody.