If everyone is truly happy, the disparity in wage might not matter.
WHo besides a total idiot would be happy making less money?
I believe Mexico was the biggest winner of NAFTA, just by offering cheaper labour and what they gained in the auto industry alone speaks for itself. In my opinion, the biggest lost of US jobs however was not to Mexico, but China: 1 to 1 for Mexico, 1 to 5 for China. Investors and big wigs were the biggest winners of NAFTA, but I don't believe the working man did so well. When someone is willing to do your job for $5 bucks an hour and work 6-days a week chances are your Board of Director will move there eventually, similarly if a large Corp. is planning on opening a plant, they are not going to open it where people expect $20/ hour. They don't care that your cost of living necessitates a $20 wages to have food and afford a roof. That's where Governments come in, they should shift more of their focus on protecting the working people of Canada and the US or lower living cost perhaps. Maybe Trump can do something, it was a promise that I'm interested in seeing if it's fulfilled!!
Except you are forgetting the part where people need to make good money to buy all these products that are aimed at the NA market. If most of the consumers can not even cover food and a roof over their heads they will not be buying junk or taking expensive vacations and then the whole thing craters.