Most of Mexico’s population isn’t European at all; it is one of the few countries where the true natives had a large role in their battle for independence from Spain (unlike the US, or South America). The ruling class (like Vicente Fox, the 6’+ white guy that was their president) are European-descended. I don’t think “Fox” is European Spanish or any native tongue from Mexico. Mexicans know that without European intervention they’d still be living like people in Papua, New Guinea; they’d have nowhere to run to to send remittances back home. Anyone in Latin America can look at Cuba to see “Life Without White People” (like the History Channel’s “Life After People”); even many Cubans can remember their Angola expedition...
Mexicans speaks a Spanish dialect in the same manner we speak an English one.
My point being that Mexico has no claim to any part of southwest US as they say. Then too, it goes back to why don't they speak any of the 63 indigenous languages in Mexico? There still are native Mexicans but then most are direct descendants of Spanish Conquistadors.