tbh central/ south america also share the same values, so to start with, let's say the american continent was turned into this mega country with freedom of movement etc, how would average citizens from usa / canada react to the tens of millions of people likely to move there due to economic reasons (more jobs/higher wage)?

with such inequality in the world and even just regional wealth differences inside countries, the reality for many countries is that projects like this see low skilled workers from rich countries have less work / opportunities , high skilled workers from poor countries leave their country in high numbers (brain drain) and other already existing issues like housing become bigger problems.