All things being equal, I might agree with you. But, they aren't--will these trucks and drivers undergo the same rigor of inspection and training that American drivers do, or will they be able to cut costs in the name of political correctness? Just as important is the fact that American drivers don't feel safe driving in Mexico, even if the laws become symmetric. Will they be subject to bribe-driven inspections? Will the police be able to protect them from organized gangs of hijackers? (We have some of these problems, but to a much lesser degree).
*Note that the Teamster-predicted carnage never ensued.