Well first you're arguing from a debatable premise, while real wages have been stagnant for some time, here is real compensation. Compensation has tracked productivity well (also see here). Wage and productivity decoupling is overstated. Also, somewhat tangentially, consider giving this a read if you have time. Second, looking at point A and point B several decades later and ascribing the outcome to a single policy is poor macroeconomic methodology. Think of the macroeconomy as a maelstrom of numerous forces and events pushing an pulling against one another, all of which realize the economy we observe. Macroeconomics uses computational models and empirics to discern the signs and magnitudes of these forces. Even if what you were saying about compensation were true, the evidence of trade as contributing to this process isn't particularly compelling. Lastly, NAFTA benefitted US workers (though it did benefit Mexico more).