1. No immigrant from Mexico or Central America is coming here because they have a better plan to fix the US, but because they're fleeing from turmoil back in their home country. However, they do provide massive economic benefits to the US, which is a consensus reached by the vast majority of economists.
2. They cannot meaningfully "fix" their own country considering how most of the problems there are caused DIRECTLY by US policy, whether that be the disastrous drug war or the fascist coups that the US implemented in Central America. The US caused the refugee crisis and now it's clutching its pearls when refugees come to the US.