If most US retirees to Mexico continue to pay Medicare premiums, and presumably pay directly for some health care and drugs in Mexico, the out of pocket cost of US healthcare above the Medicare premiums must be pretty high.
The choice US retirees have if they move somewhere they qualify for the local health insurance system is whether to continue to pay Medicare premiums knowing that if they don't they will have to pay 10% greater premiums if they ever return to the US for every year they miss. If I retire to the UK my inclination would be to not pay Medicare and rely on the NHS, but if I ever wanted to move back to the US, say in my 70s or 80s, I end up paying enormous Medicare premiums.