If they did just that, stayed in their own newly claimed borders, did not venture outside those borders, and swore off external violence, and sought to live in the Dark Ages that would be fine. If they then compounded that by refusing sanctuary to any person or group that espoused Jihad against anyone or any other nation then that would be fine.
Now do you believe in the Tooth Fairy?
Because that is the kind of thinking it would take for One to actually believe that is possible or likely.
You mean the Tooth Fairy isn't real? What about Santa, the Easter Bunny????
I didn't say it would happen, I said it would potentially be the most dangerous course of action from a western perspective. If nothing else, even if ISIS did nothing else, it's existence as an established state would be a template for and serve to encourage other groups in other parts of the world - the Afghan group HIG - a group that has even managed to alienate the Taliban through its extremist tactics today announced they were considering pledging allegiance to ISIS. Dealing with a rogue state is far more complex than dealing with a terrorist organisation as you have all the issues of governance, infrastructure and the general well being of the population to consider on top of removing the threat - all the stuff we forgot about in 2003 in Iraq.