ISIL is one of the many names for the terrorist organization operating out of Syria. This organization has now lost control of Iraqi oil. No matter what you call them, they are fast losing ground in the Middle East, which spells success for the many countries that are working to defeat them.
Yes, in my personal opinion the oil is the main reason for those terrorist alliances and gatherings. If they no longer control the oil supplies of Iraq, then I think they do not have the real reason to exist. All the other explanations of their existence are just irrelevant to my mind.
A series of military blows from a large number of enemies in the military theatre as well as the loss of vital funding from private individuals and oil reserves have been enormously detrimental to the progress of Islamic State. They have also lost a number of their key people and morale is currently low. However, the terrorist group should not be underestimated and until their grip on their Syrian bases is eroded, they should not be written off as a malevolent force in the world.
ISIS has been losing control in the Middle East for a long time now, but that doesn't mean it will be completely eradicated any time soon. There are enough people who believe in the cause that it will still be around. Even in the United States and other places around the world, we see people who had little to no connection to ISIS itself committing terrorist acts and declaring their allegiance to ISIS.