Hi Mexico Resident, I would vote for the middle course. Part of the problem is that most retires are on a limited budget, but are still used to an American style life. One of the attractions of Mexico is that its cheaper. As a counter balancing issue, services aren’t readily available (at least at an American level.)
Another issue is the rule of law. Its still quite tenuous in Mexico. Even though crime against gringos isn’t particularly bad in most of Mexico, Americans are still scared. Foreigners are always in a weaker position than a Mexican citizen in the eyes of the law.
The third big issue is ownership. Within 50k of the oceans and borders we can’t own anything. Even though my father has 50 year lease with two guaranteed renewals, its not the same in our eyes as owning.
I think that as these macro issues are worked through more Americans will move south for retirement, and some even to work and build careers. We have a very long way to go though before the trickle south matches the torrent going north.