A cooler running motor ought to last longer and a more efficient motor ought to run cooler. So doesn't higher efficiency mean longer motor life?
Not necessarily. First, if by "life" you mean the time until the first repair, that's more likely to be because of a bearing failure than because of winding temperature. Second, a more efficent motor does have lower heat-producing losses, but that also may allow use of a less robust cooling fan (meaning lower windage loss) because the