Your social security benefits are derived, in part, from how much you paid in on your top earning years. Thus, if you didn't pay in you would have very very very little social security (which is already too small of a benefit to live off of). Thus, I don't think there is a moral hazard of social security that would cause people to return to the US for those benefits. They could amend the law (not sure if it's specifics) that you have to pay in a minimum amount if you want to be paid out while living in another country. Simple fix to remove any poorly aligned incentive.

Medicare is much trickier as every American over 65 is entitled to it, but I'm not sure how that coverage works if you live outside the US.