2 Major things breakdown and affect shoe life.
The midsole and the tread. The midsole compresses and the tread, well, dissapears.
With Midsole compression, you will loose cushion the older the shoe, but if you don't need that cushion, then compression will not bother you. Plus midsole compression does not happen expodentially. It actually occurs almost in reverse of that. As the cushion compresses, it takes longer and longer to obtain the same amount of compression. Meaning if it takes 100 miles to get 40% compresssion (made up numbers) then the next 100 miles you may only see an addition 15% compression.
Some pople just don't need cushion, what happens For these people (including me) is the only thing that affects shoe life is tread. A pair of XC shoes will last a long time, where as a racing flat such as the T6 will only get you 400 miles maybe. (the tread on a T6 is nothing more then foam).
But if you due require some cushion (these are the guys that back/hips start to hurt as shoes get older) they have much different limitations.
Oh and sometimes your just rip the damn shoe, it happens alot in flats.