I don't know the right answer as to how to defend against terrorism, etc. But this argument makes no sense. "They" (whoever they are) don't need a reason to play, they already have one. Choosing "not to play" is just allowing whoever they are to do whatever they want.

Currently they think they have a reason to play. If we stop playing, they aren't going to for some length of time, perhaps measured in generations. Given that "playing" means "attacking us trying to kill as many people as possible", I do not want to be in the situation where they are playing offense and we aren't playing defense.

If the West stops playing, it's not like Turkey, China, Russia, Iran, and Saudi Arabia will stop too. And there's no reason to believe the war would become less bloody with even more refugees.