They're having delusions of grandeur, I think they've had their moment. With not even 10,000 troops under their command IS should be stretched pretty thinly right now. They may be holding a lot of territory, but most of it is desert. They are holding two large cities, Mosul in Iraq and Raqqa in Syria, but I doubt they'd be able to hold those towns if there is a rebellion against them. Mosul has 2 million inhabitants, get the people angry enough and they will kick IS out.....Chance of that happening? dunno
civilians alone can't do that, ISIS is very organized well trained and holds a solid grip on their towns controlling everything that comes in and out i.e weapons, vehicles, food etc. those civilians must rally behind one of them rebel/mujahideen groups, sheer numbers wont do any good otherwise the 100k or so fighters would've defeated ISIS's 10k long time ago. even rebels/mujahideen themselves are losing ground now since they are not united and not as determined to fight ISIS as the later is. while ISIS is reaping rebels/mujahideen top commanders heads one after another, some of them idiotically still debate if it's correct to fight ISIS or is it a sin..!!
From a military perspective, Isis thrived on the disunity of the Syrian rebels and the inconsistencies of their backers. When al-Baghdadi announced the merger between his group in Iraq and Jabhat al-Nusra, the group started to act as a state in rebel-held areas. Despite its low numbers, Isis established a reign of terror in many areas across Syria. It alienated most of the rebel groups by creating smothering checkpoints, confiscating weapons and imposing its ideology on the local population, something Jabhat al-Nusra had avoided. By the end of last year, all rebel groups declared war against Isis and drove it out of Idlib, most of Aleppo and Deir Ezzor. But the war cost the rebels a lot: around 7,000 people were killed in the battles against Isis and the main rebel coalitions started to disintegrate as a result of the fighting. The Islamic Front, once the most powerful rebel coalition, is now a shell of its former self. Jabhat al-Nusra, once the most effective rebel group, is struggling to halt the drifting of its fighters or sympathisers to Isis, especially after it lost its stronghold in Deir Ezzor.