It's not that banning people in the abstract is a terrible thing. It's that Trump chose about the most haphazard and ineffectual way to design and implement a travel/immigration ban. Quite simply, there's just no rationally supportable basis for Trump's edict.
There's
scads more immigration-related content worth reading from the Cato Institute, which is not by any stretch of the imagination a liberal think tank, but it a place where people actually think, often (not always) rationally. I don't, from a normative standpoint, agree with everything they propose, but I won't deny that their proposals are often (not always) rigorously developed.
Even looking at the ban from the standpoint of the value of lives saved, it just doesn't "add up." Consider this:
Government regulators frequently estimate how much it will cost to save a single life — often referred to as a “statistical life” — under new safety rules that they propose. The key insight behind that estimate is that human life is certainly very precious, but not infinitely so. Indeed, everything people do that slightly increases their chances of dying, such as driving a car, would be unthinkable if they placed an infinite value on their own lives.
The tricky part is figuring out how much that statistical life is actually worth. Many s
ecurity experts place a high cost of $15 million on each statistical life. [ Does a study get more bipartisan than the one at the link? It's produced by AEI and Brookings.] Using this estimate, a new terrorism prevention rule would be rational if the value of statistical lives saved by the rule were at least as high as the cost it imposed. If Trump’s refugee restrictions reduce the already low chance of dying in a refugee terrorist attack by a further 50 percent, to about one in 5.5 billion per year, then it would cost Americans about
$525.5 million per life saved in lost economic output that would have accrued to U.S. natives—about 35 times as great as the benefit. But the larger cost would be the 510,000 fewer refugees resettled in the United States than would have been without the restrictions. Those tremendous costs would buy the United States one fewer murder committed by a refugee-terrorist over the next 17 years.
Those calculations notwithstanding, not a single life would have been saved had Trump’s executive order been put in place 41 years ago. Just 17 terrorists from the seven Muslim countries included in Trump’s travel ban have been convicted of planning or carrying out terrorist attacks on U.S. soil from 1975 to the end of 2015, and none resulted in fatalities.