Awesome you got that many miles. I put City Grips on mine and the rear has just melted away at about half the miles. I'm not sure of longevity but I'd be real happy with 15,000 miles.
I'm by no means an expert on tires, but I think the speed rating would have something to do with longevity. I do know that some automobile tires are rated to last longer than others, but I don't know the coding mechanism they use to explain how that works.
I'm sure a little investigation with your favorite search engine would turn up some information on how all of that works.