Nothing will happen. Denying NAI the approval on grounds of article 17 bis without any legal basis will undermine the credibility of the open skies agreement, which, in case of a sure retaliation of the EU, would eventually deliver a big blow to the US airline industry as the US big 3 are among the main profiteers from said agreement. Not being a fan of Trump, but he and especially his advisors are intelligent enough to see through this. And this is not even accounting for the positives that increased travel will bring to the US economy.
So you and your union friends can cry and moan as much as you like, eventually the new reality will sink in.
Here is an excellent and factual analysis from earlier this year:
http://centreforaviation.com/analysis/norwegian-airs-nai-awaits-final-approval-of-us-rights-credibility-of-us-eu-open-skies-is-at-stake-283001
Some quotes:
"It seems that one of the side effects of US consolidation has been to concentrate considerable political power (in addition to market power) in the hands of the three major legacy airline groups. The US big three have benefited hugely from consolidation and from the US-led open skies movement, which provided the context for the anti-trust immunity of the powerful North Atlantic joint ventures that dominate that market.
Now faced with a new competitor developing a new business model, they have joined forces with their own labour unions and are seeking to shut the door on competition"
and
"In its analysis of 17-Mar-2016, DoT's General Counsel stressed that no public interest analysis was necessary, but "it would be very difficult to construe promoting the competitive position of United States carriers as a compelling reason to outright reject a fit foreign competitor"
Its analysis added that "it would be entirely novel and legally unsustainable" to find that this and the labour issue outweigh all the other public interest factors supporting NAI's application, including:
the availability of low-priced services;
maximum reliance on competitive market forces;
avoiding unreasonable industry concentration;
freedom to offer prices corresponding to demand;
elimination of operational and marketing restrictions;
opportunities for foreign carrier services in exchange for comparable rights and
action consistent with international agreements
"
Says it all really...