However, the overwhelming consensus of the economics profession remains that while these arguments are theoretically valid under certain assumptions, these assumptions do not usually hold and should not be used to guide trade policy. Gregory Mankiw, chairman of the Harvard Economics Department, has said: ″Few propositions command as much consensus among professional economists as that open world trade increases economic growth and raises living standards.″
It's sad to have to rehash this every time people on HN who've never studied economics figure they're experts on the subject, much like happens with climate change denialists, so I'll have to repeat it again. There is an overwhelming consensus among people who study the subject that protectionism has a negative effect on economic growth and economic welfare [1][2][3][4][5][6]. Even economists who lean interventionist like Paul Krugman strongly agree, and have tried time and time again to explain why free trade is a good thing to an ignorant public. It is shameful that so many people feel obligated to opine strongly on a subject which they don't understand, causing bad policies to be enacted.
Protectionism makes society worse off. It makes the world poorer.
[1] https://www.nytimes.com/2015/04/26/upshot/economists-actuall...
[2] https://research.stlouisfed.org/publications/review/2004/09/...
[3] https://www.jstor.org/stable/2117691
[4] http://www.igmchicago.org/surveys/free-trade