From a national security perspective, wealth is important but so is population size. If you have both, you win. Industrial production is a third factor in national security, related to wealth and population size, and Mexico can help with that. At the moment, Brazil is a non-player but in a hundred years that could change given their population size and you could have a potential Brazil-US rivalry. An integration with Mexico will be a contingency for that, giving north US a buffer zone in addition to a larger population.

From a prosperity perspective, that seven percent will grow, especially when Mexico can benefit from strong US institutions. You also have to factor in that the entire pie will get bigger due to free labor and capital flow, which will be the main win here. So it won't be 107 percent it'll be 107 percent times by a factor.

Mexico is also culturally adjacent to the US being a Catholic majority country and immigrants from Mexico have a history of integrating quickly.

Transfer payments may be needed to make the US lower classes "whole" given they're now competing with so many more people which will dilute their wages. But if done properly it should be a positive sum situation where everyone benefits.

Pan-Americanism is a fundamentally good idea if implemented gradually and carefully.

I completely agree with your positive outlook on the possibilities. Too many people are adopting a zero-sum mindset on the subject.