Oh Mary... but you lack imagination.   NAFTA was signed in 1994 and so much has changed since then.  Canada and Mexico have both grown because of NAFTA - that is true.  Losing NAFTA will cause us to change - that is true.  

But here is the thing:  technology (especially transportation),  and geopolitical circumstances (including the emergence of more developed countries/customers) have changed dramatically.  The USA is weaker than it was in 1994.  If we are to renegotiate we will do so with more options and therefore more strength.