We couldn't find any serious degradation of the upper from either our testing or user reports, but it has other design issues that might limit that longevity, especially for foot-scuffers and those that get a lot of miles in a season.
With a speed drill like the Fartlek, you want a light shoe that can move with your feet when you need to pick up the pace every 30 seconds or so.
It's hard to argue with that equation.