There is absolutely no way to determine the magnitude of the advantage Rossi had on his personal tires against others on 2nd/3rd tier material; Casey Stoner was very clear about this even in 2006, the last year of the SNS era (I cite CS particularly because he spoke about this reality specifically, and I assume you believe him. Incidentally, makes me wonder what Viñalez might say today if he were willing to speak his mind) Alas, it is a plausible viable position then, using your assertion, that SNS may have hindered Rossi's run of titles! It is a 'reasonable' position to argue! The conventional wisdom is flipped, instead of perceiving all these years that exclusive personal tires advantaged Rossi, perhaps all this time he was frustrated. More over, through the convention of rules book, regardless of advantage or disadvantage, it is 'always' "fair". Ironically, Rossi himself blamed poor tire performance for his inability to "win" the championship. Either Casey Stoner a liar or the SNS did in fact present significant advantage to Rossi, "fairly". One interesting question to ponder is not how well Rossi would perform on SNS, but how he might perform on 2nd/3rd tires against those on tires they personally preferred. If my aunt... Except there are examples in Rossi's career where we can reasonably extrapolate what might have been, namely his Ducati stint. And, Rossi's resurgence post-Bridstone coinciding with the introduction of Michelin. Nonetheless, the discussion is futile.
I've read you say this often, and I've marveled the same every time to be honest. I don't get it. But, I'm learning this year, particularly in light of our political climate, that 'all' debate is futile. We're all going to 'believe' what we believe, in the face or the absence of evidence, logic, reason, probability, and simple connection of dots. The ocean is made of water, no it's not! The sun is hotter than pluto, no it's not! Rossi had personal tires made for him to compete against others on far inferior material which resulted in unreliable, exaggerated, skewed, outcomes, no it didn't. The SNS helped Rossi, no, they hurt Rossi! It is a "fair" position to take.