The biggest loss for USA will be ability to attract and cultivate brightest talent from all around the world. The populist/nationalist movements always accuse outsiders/diversity as reason for the economic adversity.
Adversity and diversity do not go along well. When things get difficult (and we all know, they are difficult in middle-america), ignorant people tend to easily believe in the actively sold lies (from the bigots) that “others” are the enemies.. and people in populist movement ignore the basic facts that
a) their competitive advantages/well-being/livelihood is not stolen by immigrants/people-who-don’t-look-like-them but shifts in technological advances employed by their own kind. The Street, The Valley and The Walmarts (category, not the company) are pre-dominantly white and male.
b) racism is not patriotism/nationalism, specifically for USA (Unlike other countries, racial identity was not pre-requisite for the American dream).
Needless to say, it’s the talent (poached from rest of the world) that made a significant difference in the innovation centric country like USA. America closing doors on smartest of the world, will only make it worse.
The issue we have is with significant income disparity and capability disparity between two segments of the society. Only addressing these disparities, by bringing the innovation and service economy to middle-america, we can eliminate the divide. Otherwise, this is the kind of disparity that leads nations into (even if only psychological) civil wars. No nation at (even psychological) civil war can be a superpower.