Well, first of all you have to find out why tires do have grip.
This is not such a stupid question as it may seem. For me, it is very hard to visualize tire behavior: I still cannot understand well basic concepts like "grip angle", so figure... I mean: I know the definition of grip angle, but I do not understand why it has to exist, even if I can "feel" it when I drive. At least I think I do...
I am aware of Mr. Persson definitive work on tire performance, developed last year. From news releases, his colleagues say that the set of equations he developed is in good agreement with measures. Please, if you are reading this, try to understand that I really have not the slightest clue of why tires stick to ground. Besides, I do not know any public references for a layman like me.
But, what I understand is this:
First: rules try to guarantee that the tire is made of one compound. You cannot have layered tires. Besides, it would be pointless to put "the good stuff" in the surface. You could put it everywhere, and that is what the rules tell you. This hardly seems a reason for changes in performance.
Second: it is very clear that a worn tire (even slightly worn) behaves very different from a new tire. This has to be explained by any theory of tire performance.
Third: tires are made of rubber, a polymer. Polymers are like trains of molecules. The "standard" wagon in this train, or smaller sub-unit it is the monomer. The quantity of wagons is variable from one molecule to another, like trains of different lenght.
Like trains or ropes, polymer molecules tend to be long and thin. Any polymer is, in my mind, composed of intertwined ropes of different length. If you imagine yourself small enough to be able to distinguish individual molecules in rubber, you could imagine that in the surface of your car's tire you will see a thick carpet made of fibers, instead of the black shiny surface you see right now.
MY hipothesis: rubber adheres to asphalt through interlocking between polymer molecules and irregularities in asphalt (asphalt being completely solid, for starters, even if it is another polymer). You could think of a supermini Velcro: the fibers or "soft side" in the Velcro is the rubber. The plastic hooks in Velcro's "hard side" are the asphalt. The fiber side is composed of fibers of different lengths.
Is evident that a soft side made of longer fibers will adhere better to the hooks. But you are "cooking" the tire, heating it through friction. This causes the "trains" int the polymer to start breaking. Those shorter fibers do not tangle equally well with the hooks.
Besides, you see that the rubber start to get brittle. It desintegrates more easily: its internal "ropes" have been broken, so they have less bonds with each other. This have another effect: the tires work by "breaking apart". The maximum energy you can get from a tire is when is "squealing", not "squalling" or "
You also can imagine