Recumbent chains are longer, so theoretically the forces that wear out a chain should be spread out, too. And that 'sort of' works in real life too. But your starting premise of an upright chain lasting 2000 miles is simplisic. Chains last as long as they last. I've never had an upright chain that was worn out after only 2000 miles, although rust ruined one in that amount of miles. OTOH, I've had recumbent chains last 6,000 miles and I've had a few last 10,000 miles before having enough wear to justify replacing them.