Let’s first look at some similar processes.
Viruses that kill everyone immediately are eventually not very successful in spreading. Persevering longer are mutations that will let their victims live enough time to infect others first. The first ones were more lethal; the latter ones will do more damage in the longer run.
Also societies tend to work like that. Capitalism is so bad, it would soon run out of people willing to work under it. But then it modified by adding Democracy as a veneer. And when the People’s voice became too powerful, snacks and TV (bread and circuses) and lying to the public were added, and the whole thing is going like a bomb.
Racial eugenetics, Nazi Germany and WW II Japan were defeated, but all over the world we still find groups and individuals, who dream of regaining power and putting to practice hatred for people of certain lineages.
ISIS is so cruel and sadistic that in the beginning most people, including most Muslims, couldn’t believe it to be real. It was regarded by many an American hoax. What religious people could enjoy mass killings of innocents? Now we all know that reality is more brutal than fiction.
However, ISIS’ troops and leaders will all find their deaths soon. But what will happen to their ideals and the thousands they trained and morally broke through making them commit sadism, rape and murder?
These “ex-soldiers” will pipe-dream on and try “milder” versions that would look more decent and be more presentable. They will not disappear into thin air. No alliance of armies or world-wide incarceration can do away with all of them. Rather, their blood-thirsty ideas will plague us for a long time still.
Defeat of ISIS will not enable lifting airport safekeeping and all the other security measures that many countries are now learning from Israel. The surviving lower-rank fighters’ disappointment and humiliation of losing the illusion of an easy and quick path to please G-d, will spark in them more fanaticism and longing for the good olden days in which killing “infidels,” submitting women and murdering gays was normal and glorious, and Muslim domination of the world finally seemed within reach. Forewarned, forearmed.