You do make sense. I agree that MM and Griggs did have to settle with the double ended rubber bushing to allow some play in the rear. But, I'm also sure that they modeled a double spherical arm and did testing too. In a perfect world, all Mustangs would come with independent rear
suspension that would stand up to the same abuse that a solid does. Even if that was so, the Ford independent rear (as it is now) is not much better than a fully built solid rear.
The inherent problem lies with the fact that we drive around on a 40 year old chassis
that wasn't designed to handle in the first place. It was designed for "Mom and Pop's" car and made to be smooth and a land boat. One does have to give credit to Griggs and MM in the sense that they're able to design
suspension parts for our aging chassis that actually work, and do a pretty damn good job too.
I think if one ran a double ended spherical upper arm, over time the upper mount would rip off due to the added strain of nothing giving in the
suspension. If you look at drag racing (I know it's different, but still exerts some of the same forces on the
suspension that corner carving does), guys that run solid (poly) bushing upper and lower arms rip apart their torque boxes unless they weld/reinforce them. I'm sure that we'd see the same thing, although not as quickly, if we ran a poly/shperical upper too.
I'm just ranting...I should get the T/A, remove the uppers, and promptly place them in the trash can and be done with it