Marquez doesn't have the longevity factor, plus, there's a new marketing paradigm shift waiting for Rossi to retire.
Rossi has had it easy street adding years to his racing with the effect of artificially inflating his record. I doubt Marc can burn long enough. Not only has Rossi competed with hardware advantages like tires and top tier factory bikes, etc, but most importantly, the willingness of the organizers to move heaven and earth in his favor (too long to list here). Not to mention the psychological effect of being the undisputed hero in this long running narrative.
I remember reading an article over 10 years ago by Michael Scott considering MotoGP life after Rossi. This was when Ferrari were looking at Rossi, at the time it seemed the natural course, and natural curve to the end of a career. Yet here we are a decade plus later. Why? ANSWER: Because Dorna learned selling the modern narrative: Rossi vs Evil Pawns makes for a very profitable marketing template, Rossi stayed on with the promise of being Dorna's perennial hero in this made-for-TV drama. It's been very successful for Dorna. However there is an underbelly to thid marketing scheme that requires villains to be sacrificed, who cares these are real humans, dumb .......s. All great Hollywood movies need villains for the hero to slay.
VR will likely ride in 2019, maybe even 2020. VR gets to write his own ticket, the power structure is something of a symbiotic relationship, he stays, Dorna get rich, win win. In the same way Yamaha were beholden to Rossi after the Debacle of 2015, despite clearly being in the wrong, nobody could at that point challenge VR's status. How powerful is Rossi's brand? For perspective, Yamaha had to have understood and was willing to sacrifice the best opportunity for a repeat championship in order to appease the monster. Yamaha were lucky to get Viñalez, though if Suzuki had any commitment to contend no way they let their contractual option to keep him lapse. To be fair, Rossi's brand will provide Yamaha value for years to come. In fact, Dorna have already declared holding indefinitely a grid slot for Team VR46.
The years of marketing Yellow vs Evil has created a robust Cult that isn't going away for many years. These minions can be easily and cheaply retained, waving their yellow flags for Team VR46 vs the World.
Even Dorna learned they had created a monster when they were accused of being part of the Spanish mafia that conspired to thwart Rossi's 2015 title. If you're curious, I'm sure you can find a video of a flustered Carmelo Ezpeleta trying desperately to distance himself from the wrath of accusations from the Valiban. It's why he put lipstick on a pig and made the old Race Direction the scapegoat; instituting a supposedly "new" Race Direction. Carmelo claimed the new and improved Race Direction was not associated with Dorna, therefore, in a slide-of-hand, any unpopular decisions by this "new" Race Direction wouldn't be a liability to Dorna's standing in the eyes of the powerful and profitable Valiban. Because Dog forbid their value be diminished from petitioners seeking to take out their rage by witholding ratings.
Dorna are clearly planing for the future, the one king vs pawns marketing model is drawing to a close and now they're turning to a 'multiple contenders' model. Once Rossi retires, Dorna will implement performance leveling like it's Nascar, effectively deploying "competition yellow flags". Dorna has been actively pushing to get that 10th title, in doing so it's extended artificially the use-by-date of Valentino. The value of keeping viewers tuned in is still paying off. However, it was reported that Dorna considered it a major problem for the 2016 title to be wrapped up so early. Contrast this to the years Rossi wrapped up the title early, no adverse reaction in sight.
Marquez will compete in an era where performance leveling will be the norm. He will be chasing an inflated record from a bygone era. Marc will certainly win a few more championships, but I doubt the days where one rider dominated the season will continue.
If you live in a glass house don't throw rocks.