When someone legally immigrates here they are considered a "permanent" resident. There are some categories of non-immigrants who can work here but not many. If a permanent resident works his 40 quarters, pays in and qualifies he should be entitled to his benefit as long as he stays in a "permanent" residence status. If he chooses to abandon his legal residency and return to his home country he should get nothing. What part of "permanent" don't people get.
The only people who should receive SS oversees are US citizens. If the immigrant wanted to receive SS and return home to retire he should have naturalized when he was here.
Illegal aliens should get nothing, I view any money they pay into the system as a partial offset for all the services they received while here.
There is a larger picture in all of this. The concept of US citizenship is being eroded to the point where it is meaningless. The difference between legal and illegal, citizen or alien is being blurred intentionally. If you can receive all the benefits citizenship, why naturalize?
The pact would be the latest, but by far the largest, of a series of treaties designed to ensure that people from one country working in another aren't taxed by both nations' social security systems.
The Mexicans argument here is a red haring and based on the false assertion that Mexicans pay Mexican SS taxes while working in the US. They don't and never did.