No-one can know this except for michelin as someone said earlier in the thread, maybe not even rossi. I don't think anyone including jumkie has claimed any of rossi's titles were due to the saturday night special tyres ; rossi's serious rivals all had them too, although the possibility that catch 22 applied and you needed to have the tyres to be a serious rival exists imo.
I don't think it is totally unreasonable to suggest that a tyre manufacturer might develop in a direction that suits one rider or bike more than another though; I seem to recall suggestions that michelin were developing for pedrosa more than rossi in 2007, that bridgestone made no pretence that
ducati were not their priority up to and including 2007, and that ducati were prepared to go with michelin going forward from 2007 despite them apparently being so hopeless. I also actually agree with j4rno that the current bridgestone tyres suit lorenzo more than rossi, whether or not by design, and that applies to stoner as well which he did not say, and this is not surprising since it has frequently been said in reports neutral to the topic of advantage that both had extreme tyre preferences.
To return to an old and now irrelevant fight, except for it being the genesis of the topic of advantage from my point of view, I do recall large volumes of posts about stoner having an advantage in 2007, with many of those arguing citing his lack of success previously including 2006 ; however surely it is beyond dispute that rossi had a tyre advantage over stoner in 2006
Btw I actually don't have a problem with riders or teams developing an advantage; as you rightly said recently that's prototype racing. It is certainly my hope that hrc will give stoner an engine /bike advantage next year
.