Do you know what the current population of the US is compared to 100 years ago? And yet, we're much better off now.
Economics is not a zero sum game.
https://en.wikipedia.org/wiki/Lump_of_labour_fallacy
Here are a few companies founded by immigrants or their children:
http://www.forbes.com/sites/stuartanderson/2011/06/19/40-per...
It is almost universally the case that when someone says "economists think X" they actually refer to a group of academics belonging to a specific school of thought which defends that particular position. Not that scholarly consensus does not matter, just a reminder that in certain fields those questions become so highly ideological that you simply cannot treat them as a math question on which you can definitely rule one thesis or another.
Opinions like this is what caused Brexit to occur, and what will probably get Donald Trump elected.
Since it's a positive sum game, you could share the gains with the few people who lose out from immigration or trade, or automation (which is probably more important than the others) and still have some left over.
Or yeah, you could get all protectionist and watch the economy whither.
And if it does whither, that's a choice the US citizenry might be satisfied with.
Right, it'd take a while.
[1] http://elections.huffingtonpost.com/2016/forecast/president
Are you 12?