Again, if we are considering the original comment I replied to about medium-term vs long-term, a move offshore to Mexico will always make sense in the long-term unless Mexican labour becomes as expensive as USA.
Considering break even points it will always make sense to move out, reasons to not move are the local incentives in the form of tax breaks and invested capital in factories, if the consideration is only about labour (which the take on unions making it easier for the move) it always make sense to move, regardless if it's a unionised workforce or not. Non-unionisation only delays this inevitability by allowing the company to milk the employees for a little longer, in the long-term it will eventually pack up and move if the incentives are not maintained (again in the form of tax breaks or if the company has recently re-invested in existing facilities).
To me, again, the fear of globalisation's side-effects is not a good excuse to promote a lack of worker solidarity, it only makes workers more powerless since at some point in the long-term the break even point is reached, without an union there's no collective force to pushback a company's decision to move their production somewhere else with cheaper labour, at least with an union there will be a conversation and negotiation around it.