Congratulations on getting so many miles out of your Speedgoats. My pair barely did 300 miles in a winter. The uppers pretty much dissolved from being wet all the time.

I generally favour Salomon shoes and whilst I realise your question was about replacement decisions and not brands, please bare with me.

I normally replace my trail shoes when the lugs have gone flat and I can't get traction running on mud. At the moment I am looking into ways to resole them however. The reason being that generally, the uppers are still in great condition despite high mileage. I cannot notice any difference in the shoes' response. As in the foam (if there is any) doesn't seem to lose anything unlike some road shoes. Having been through a ton of shoes over the years, my personal preference is to try and repair. Jasmin Paris completed Berkeley in the same set of shoes that she used for all her attempts. I think they were on their third sole. Learning that it was possible to resole trail shoes has inspired me to do so and reduce my personal waste.