Dr Mahmoud Othman, a veteran member of the Iraqi Kurdish leadership who recently retired from the Iraqi parliament, said there was a misunderstanding as to why Gulf countries paid off IS. It is not only that donors are supporters of IS, but that the movement “gets money from the Arab countries because they are afraid of it”, he says. “Gulf countries give money to Da’esh so that it promises not to carry out operations on their territory.”
... It has drafted a list of draconian punishments for those not willing to fight, starting with 80 lashes and ending with execution.
... Isis has also lowered the conscription age below 18 years of age....
... In the most optimistic scenarios Isis splits or there is a popular uprising against it, but so far there is no sign of this and Isis has proved that it exacts merciless vengeance against any individual or community opposed to it.
Patrick Cockburn also explains how the Shiite Iraqis and the Kurds aren't coordinating/cooperating enough.
Egypt and Jordan have entered the fight. Western nations have been openly discussing entering it too. However, without "boots on the ground" and a sustained strong-armed occupation on the order of post-WWII Germany and Japan, the US plan is extremely weak.
Giving Russia and Syria's Assad a hard time right now only strengthens the Sunni Jihadis (IS). It's way beyond the time for the US to stop bickering with Russia over Ukraine, stop attacking Assad via iffy proxies at best, and stop harassing Iran over a non-existent nuclear-weapons program.
See also: "If They Oust Him, it Will Only Get Worse. "US, Turkey seeking to oust Assad" - YouTube" and "West Asserts IS (ISIL, ISIS, Daesh, Da‘ish) is Heretical, but That's Ignorant, False Propaganda."
Then there's this observation from Joshua Keating:
As political scientist Ora Szekely writes in a new analysis for Reuters, there are signs that the group’s economic model may no longer be sustainable. With no government patrons, ISIS has to raise money itself. It’s done this by selling oil, ransoming hostages, and looting and extorting the people who live in its territory. Oil revenues are believed to have dropped due to the U.S. bombing campaign. As for ransoms, ISIS may have already ransomed or killed off most of its high value prisoners. With Kayla Mueller’s death, ISIS is now believed to be holding only four foreign hostages, down from 23 last August.
ISIS could collect more taxes and property from locals, but that risks driving them to rebel as they did against the group’s predecessor in Anbar during the last U.S. war in Iraq. The caliphate is already facing a looming food shortage, as many farmers haven’t planted their wheat crops due to lack of access to equipment and fertilizer, which could deal a further blow to the group’s legitimacy as a government. Additionally, recent coalition gains in Iraq, though not significant in terms of area, threaten to cut off ISIS’s supply routes.
So, it's a very mixed bag with IS's support rather debatable. Nevertheless, the US certainly dragged its feet. Plus, Turkey has been no help against IS but rather supports Sunni sharia (as opposed to Shiitism) without being very overt about it for fear of becoming more alienated from Europe and others.