Not all consequentialist arguments – slippery slopes, where one event is predicted to lead to a chain of subsequent and presumably worse events – are logical fallacies. I can assert, for example, that an individual making a terrorist threat to O'Hare Airport would begin a chain of events that would disrupt air travel throughout the United States. It would likely cause delays, cancellations, or even closure of an airport through which thousands of flights connect for passenger transfers every day, so O'Hare is infamous for creating "butterfly effects" in air travel. The slightest hiccup there is felt everywhere. I can't predict with any certainty what specific effects will follow the initial action, but I can construct a solid, evidence based argument that there will inevitably be consequences.
I point this out to underscore that when the facts are taken into account, banning Muslim immigration into the U.S., as Trump and many of his fellow travelers advocate, will be only the first step in a chain of escalating actions against Muslims. No one can predict with anything other than pure speculation as a guide exactly what those next steps will be, but there is zero doubt that there will be "next steps." We can conclude this with confidence because it is so simple to demonstrate that the first step – selectively banning Muslim immigration – will not accomplish its goal of eliminating terrorist attacks and making Americans "feel safer."
Only the most superficial understanding of the evolution of terrorism over the past four decades is necessary to understand that the age of Terror as Spectacle or terrorism to achieve political ends is over as far as the United States is concerned. Past terrorist groups either committed acts of terror to accomplish a specific goal – i.e., hijacking a plane to secure the release of prisoners – or to create a grand, publicity-seeking spectacle – Black September, the Dawson's Field hijackings, the Japanese Embassy hijacking in Lima, and so on. Al-Qaeda took the idea of spectacle but redirected the goal toward mass killing. They were drawn to the symbolism of attacking icons while also racking up a large body count. ISIS and ISIS-inspired terror, though, is the next step in that evolution. They've done away with the big, expensive, complicated (and therefore less certain of succeeding) spectacle in favor of the low tech, high body count approach. Why concoct some intricate plot to teach people to fly planes into buildings when you can have someone rent a truck and drive it into a crowd? Why not just grab a gun or strap on an explosive vest and head down to the mall? Why recruit expert bomb makers when any yahoo in the world can buy a Walmart crock pot and download a simple bomb schematic from the internet and construct it out of supplies you can buy at any hardware store?
The tradeoff is in glamour and efficiency. Each individual attack will result in fewer deaths than a single grand terror attack, and certainly there's none of the prestige and feeling of superiority that comes from crafting a complicated plot to "defeat" the intelligence communities in western nations. Cumulatively, though, the body count evens out and the attacks make up for what they lack in efficiency with low cost, ease of planning, and quantity. Most importantly, they are virtually impossible if not literally impossible to stop. One person, perhaps having read some ideas off an internet message board, perhaps inspired by an individual who contacted him online, or perhaps just acting on his own limited but sufficient imagination, can easily concoct a plan to kill a bunch of people. Anybody who is of the mindset to do so can rent a truck and drive it into a crowd, go to a crowded public place and start shooting, or construct a crude homemade explosive device. A plot like the 9-11 attack took years to plan and execute and had enough people and moving parts involved that the opportunity for law enforcement to intercede was always there. If a guy or a small group of people decide they're going to go shoot up the mall or the soccer stadium, nobody can really stop them from doing that provided they have enough discipline not to give themselves away beforehand.
So, accepting the reality that the new terrorism is low-tech, crude, cheap, and therefore unstoppable, what is likely to happen after we ban Muslim immigration and find that it has not made us Feel Safe? When the terrorist attacks that require little to no imagination, preparation, or ability to carry out continue, are the same political forces that demanded the "No Muslims" policy likely to throw up their hands and say, Oh well, we tried? Or are they more likely to ratchet up their rhetoric and their proposed solutions as they chase the dragon of Feeling Safe in a world in which complete security is a goal that can never be achieved? We can't predict what will come next, but we can predict that something will indeed come next once we have tried an immigration ban and discovered – to the surprise only of people oblivious to the world around them and unable to understand modern terrorism for what it is – that it did not end terrorism. What happens when we're five steps down the slope – say, having rounded up and interred every vaguely Middle Eastern looking person – and we find that even that hasn't stopped this kind of terrorist attack that can't be stopped? I have no clear idea where the imaginations of people like Trump go, but I have a clear sense that I do not want to learn by experience.
The flaw in the things xenophobic, anti-Muslim, and nationalist elements in our society propose to protect us from terrorism – moral, ethical, and legal problems aside – is that they will not actually protect us from terrorism. The process of watching them increase their dosage in pursuit of a high they can never achieve is not something we can describe in any detail until it happens, but we can predict safely that it will be unpleasant.