can we talk about the option of offering discount incentives for people who lose weight?
How about discount incentives for people who commit to certain healthy lifestyle choices (e.g., exercise programs, nutrition programs, smoking cessation)? I think these present some of the same risks you and I mentioned earlier, but at least they seem more fair and rational than basing everything on weight, which is a highly fallible proxy for health.
It already exists. United Healthcare reimburses you up to 200 dollars every 6 months if you make 60 gym visits and have your gym verify it.