For one ISIS is continuing to go on the offensive in areas that they know they can never hold onto. Their attempt to take Kobane was/is a complete failure. They thought it would be easy given the poorly armed YPG and of course their pals in Turkey would keep any Kurdish resupply attempts from happening. The one thing they didn't factor into the equation was consistent US air support in kobane. The constant bombardment from the sky has demoralized ISIS fighters who were told they would have an easy fight over the "secular Kurdish dogs". They put a ton of effort into the fight by moving armor and tanks they stole from Mosul to kobane which ended up being destroyed by U.S. bombings.
So far what we know about their western recruits is Thatcher are with mentally unstable or incapable of adapting to a normal society. These aren't exactly prime candidates for building a new society. The majority of affluent and well educated Syrians have fled the country. This leaves a large void for engineers doctors and other people that have the necessary experience and skills for nation building. So essentially you can't build a country with a bunch of kids that consider murdering fellow Muslims as a great summer vacation.