dyh,
Durability is completely individual. The 400mi advice that comes out of running stores is BS. I know guys that have put 1000mi+ on RC-150s. In my experience, runners who run in something minimal will get far longer use out of it as opposed to someone running in regular shoes.
Why do shoes like the 2090 "die" or do people feel like they die? Because the EVA that composes the midsole of the shoe is like a bunch of doughnuts all packed together and the idea is that you apply pressure to the doughnut and it compresses and once you step off the doughnut rebounds, and after sometime, the doughnut stops rebounding all the way and is generally pretty flat. You can have someone notice this by putting their old shoe on one foot and the new one on the other and notice the difference in height.
But who needs cushioning? The whole industry is based on the idea that you risk injury if your impact is not properly dampened. New Balance just went and f***ed up their best shoe under the thinking that a thicker midsole would lead to greater durability. Where does that idea come from? If you had a shoe 48mm thick in the heel, whould it last "twice as long"?
Why is it that every runner I know that runs in full time in something minimal like a flat, xc shoe or modified track spike can run on them until the outsole wears off, while the 2090 and Kayano wearers are hurting after 500 miles? Because that massive heel and all that crap and insulation under the foot do a lot of things to that foot to make it highly inefficient. You lose kinestetic awareness of the ground, turn off you proceptors which help stabilize your foot and adjust it to varying surfaces. You shorten achilles, soleus and calf lengths which create a dependency on that high heel, and by rasing the heel, you've limited the mobility and therefore impact handling properties of the ankle, so now impact cannot be dealt with at the ankle, as intended, it travels up the lower leg to the next available joint, the knee. So as soon as that pad under the heel wears out, they start to feel it real quick in the knee. If you lower the heel to the ground, there is no need for all that padding -- it creates its own need.
If you ran 100% of your running barefoot, would you ever need to change your feet? Of course not, your feet are designed to run for a long time. Bushmen in Namibia will chase down zebras (barefoot) for up to 6 hours until the zebra tires and stops for some water and then is ambushed. The idea that shoes are necessary is ridiculous. Why would humans be so poorly designed for such a necessary activity? It's akin to putting people in wheelchairs because we aren't meant to get around wth some measure of support, Why is it that shod people are far more likely to have an injury than an unshod person? Why run in a high heel? No one has given me a good answer on that.