An individual choosing to ‘post-retire’ in a lower-cost location, I could understand.
But the US shipping its indigent elderly to a third-world holding pen until death sounds absolutely horrid.
Lots of Americans already go to Mexico for health care reasons. Getting the same surgery from competent surgeons for $10-30,000 less is very appealing to a lot of people. Often American surgeons will go there just to perform surgeries without having to carry millions of dollars in malpractice insurance.
Add to that there are already a bunch of major retirement communities there for Americans, some specialize just for Canadians. Though estimates of how many Americans live there full time are inflated, about 125,000 is a reasonable estimate.
Unless you specifically choose a place that is rural or rustic, a lot of these American expat communities are very modern and comfortable, with lots of swimming pools, golf courses, green landscaping, etc.
Bottom line, Mexico isn’t the Moon, and while there is a lot of poverty in Mexico, if you don’t want to revel in it you don’t have to.