You're assuming that humans will remain healthy in low gravity. We already know they deteriorate rapidly in zero gravity.
NASA's consensus has been for decades that ANY gravity is better than none. The body atrophies up to a point, but only insofar as it eventually adapts to the minimum fitness needed to survive in its particular environment (your bone and muscle density is reduced to that which is actually needed in that gravity).
The flip side, of course, is that people who are adapted to lower gravity worlds -- the moon or Mars, for example -- wouldn't be able to tolerate full Earth gravity without months or years of physical therapy. People who are BORN in that environment couldn't even do that much. They'd be perfectly healthy right where they are, but they wouldn't survive half an hour on Earth.
Not that's really a problem, since the flow of emigration is unlikely to be two-way.