*For now - as you say, that's a NAFTA visa and some professions are already having issues crossing the border (nurses and medical professionals lately).
It's very unclear what will happen if talks don't lead to another deal, and I don't think most small start-ups could handle the fallout if they all of the sudden need to convert their Canadian staff to another entry mechanism.
Not sure why you got downvoted there, you raise a good point about TN and NAFTA (though your assertion about nurses having issues at the border cries for a citation). I don't think you're wrong that people should be concerned about the current NAFTA negotiations, especially if they are Canadian (or Mexican) on TN or employ any Canadians or Mexicans on TN.