I was thinking that as I wrote my last post, but being in the running for the title of GOAT I think does require to be in the rarefied company of multiple manufacturer titles (IMHO of course). I suppose it stems from the saying, 'if you're fast, then you're fast on anything.'
That's not to say that the guys who stuck with one manufacturer couldn't have won with another, just that those who have done have proved their adaptability.
I reckon this is probably a thread all of it's own.
While I totally see where you are coming from (and yes, it may well be it's own thread) I would counter-argue that the modern racer is equally adaptable on a single manufacturer machine, if not better than many racers who may have swapped machines across the years.
I say this as today from year to year rules change that require a new learning curve, be that weight of the machines that seems to constantly change year on year, spec tyres, spec imu, changing fuel restrictions, engine allocation and so forth. In years past each rider (at top factory level) did not need to concern themselves which such things as there were no restrictions on engines, weights were constant, rules did not change significantly year on year, tyres provided a greater selection which allowed manufacturers to build for a brand or even a bike without constraints that it need to work for all.
Yes, some will say that the spec equipment will be factored with certain riders or results in mind, something that we see often and in many regards some aspects of which are still debatable.
It is an interesting discussion actually and each argument has pro's and con's and likely may be influenced by how long people have been watching as well.
On a personal level, for me I am a hypocrite as I am a Doohan tragic and he did not test himself on another brand, but I also saw what he did to world class superbike fields on Yamaha and Honda superbikes which kind of allays concern of adaptability (same goes for many of the same era - I have to make excuses). But at the same time, I also partially subscribe to the 'test yourself on another brand' as well, but for me that has a lot to do with my desire to see the rider on a different brand .........