If it's that easy why hasn't it happened yet?
There's something missing in this scenario.

We have not yet reached that state of mind required to wage total war against these villains. Whether it will take a catastrophic event such as described here, or a steady rivulet of "one off" seemingly random terror attacks, one day it will come, or we will perish.
Somehow, we were able to firebomb Dresden, Hamburg, Tokyo, and countless other places without anyone saying "That's not who we are". No one has ever succeeded in making a cogent argument that other, less severe tactics would have sufficed. We don't think less of ourselves for having done it.
We are still here, which tends to mitigate any vestigial pangs of conscience we might feel.
The people who tried to killed us are dead.

Lets send some of our people to the muslim countries and give THEM a little terror

I'm not a tinfoil hat wearer, but I really don't think that the San Bernardino shooters went to all that hassle with guns, pipebombs, money exchanges, shadowy dealings with other "Arab" men and giving their newborn child away just to shoot up the office Christmas party. I think that they were part of something bigger planned for Christmas and the pinhead got so angry that he and that glamour queen wife of his went off early.

This isn't over.

Yes, San Bernardino is part of some bigger imminent thing. Both BO and Trump have signaled their expectation of that in the past few days.

BO in particular with his ghastly presence reflects his certain knowledge. And when his heart's delight occurs, BO wants you to know you are to blame.

Folks are also pre-blaming Trump for the coming Christmas massacres, the "holiday" Beslans. Trump's fake hair will have sent the barbarians over the edge, the hair plus his magical rhetoric which changes peaceful people into blood-thirsty maniacs.

Here is something we all know to be true: if the NYC scenario happened, Trump's proposed temporary ban would would be the very least acceptable action to the vast majority of Americans. Much more will be demanded by voice vote.

So why isn't the plausibility of the scenario enough to justify Trump's limited proposal now, before it happens?

"He comes out of the subway and bides his time at the McDonalds on Union Square with a fine little Egg McMuffin."

Ah, Please hold the Canadian bacon on the Egg McMuffin!

Unless the United States has a complete change of government, the A-holes running the show are perfectly willing to absorb the consequences of any attack from the "religion of peace" unless they are the ones being slaughtered.

Americans are trapped between Islam and the American government. They are in league with one another.

As a former resident of L.I., I theorized a scenario wherin a box car containing a nuclear device is shuttled under the East River along with other freight. The LIRR still runs freight trains from L.I. in the early morning hours. When the train is in the tunnel, underneath the river, the device is detonated rupturing the tunnel and floding the entire subway system. I hope it never happens.