Hmmm. all modern street motorcycles including and especially the highest performance types, Hyabusa, etc, and GP racers have had radials for decades. The problem you indicate has been fully addressed and radials are the way to go... IMHO and that of all manufacturers, high mileage/performance enthusiasts like the Iron Butt Crowd, and racers.
From the newest research I have, the radial fix for this problem was to add bias plys, thicken the side rubber and harden the rubber compound of the sidewall. That is, make the radial tyre act like a bias ply tyre. That's what I meant by kind of defeating the radial design concept.
If the hybrid radial-bias motorcycle tyre is anything, it is strong. The extra plys and harder compounds add durability, and the sought after wider stance adds traction. This combo is especially good on large cruiser types with wide tyres that do not lean hard into corners, but get lots of highway miles.
Also, a factor I didn't mention earlier was aspect ratio. As the aspect ratios get lower and lower, the sidewall becomes less prone to over flex, simply because there is less of it to flex. Again, good for modern bikes with wide low tyres - bad for older bikes with narrow tall tyres. (I'm playing a little of the devil's advocate here - as I believe tyres are way overengineered for their purpose, so regardless of these considerations, radials are more than adequate for our bikes).
BTW...pinching the sidewall, creating an excessive pinch angle (when rim is too narrow for the tyre) is bad because it increases leverage force on the sidewall to move outward. But, pinching a radial sidewall is extra bad if you believe the sidewalls are inherently weaker. Also, the pinch angle becomes a bigger headache as aspect ratio drops. i.e. rims must get even wider to keep up with lower aspect ratios. Hence, from both perspectives, the need for wider rims.
