The future of the ST? I used to think that as long as there is the standard Explorer, Ford would make a Sport Trac version of it. But the market is changing drastically and truck/SUV sales definitely aren't what they used to be. Ford is hinting that future Explorers will be different, as in unibody construction and what not. Plus there's the CAFE requirement of having an average of 20mpgs (yay). At any rate, I dunno if Ford will make a Sport Trac version of the next generation Explorer (if there is a next). Maybe Ford will finally redesign the Ranger and offer a crew-cab version of it.
As for your other question, I don't know of any truck that I would like to drive 40K a year. I mean I would gladly do it in my Trac, but not with its 14.5 mpg average. And show me a V6, let alone a V8, truck out there that will average better than 14.5 mpgs..realistically. I do have high-hopes for Ford's upcoming EcoTech engines, which claims a V6 with V8 performance and drastically improved mileage!