U.S. intelligence believes that since September 2015 ISIL appears to have lost 25,000 fighters in combat (mainly in Syria, Iraq and Libya). . . . about 45,000 ISIL fighters have died since 2013. It’s believed that ISIL currently has only about 20,000 fighters available, mostly in Syria and Iraq. There are a few thousand more in northern Libya, eastern Afghanistan and Egypt. In all five countries ISIL is under heavy attack.From Strategy Page.
Air strikes, about 75% delivered by U.S. forces, have been pivotal in destroying ISIS military equipment and bases.
Of course, ISIS (a.k.a. ISIL a.k.a. Daesh) have also inflicted horrific casualties on its targets.
For example, today's paper notes that the Associated Press with the help of local reports and satellite imagery has identified 72 mass graves of people killed by ISIS in which 5,000 to 15,000 people are buried, including almost all members of one tribe in the region, and large numbers of members of the Yazidi religion whom ISIS has attempted to genocidally exterminate.
ISIS has lots significant amounts of territory during the campaign from its peak, but the Syrian part of the conflict is profoundly complex with many players who have mixed loyalties. Also many of the predominantly Sunni Arab people subject to ISIS rule are skeptical that the Shi'ite controlled and Iran allied Iraqi government is a desirable alternative. People without good alternatives keep fighting for their side until a good alternative materializes, even if they are clearly outmatched and will suffer many casualties as a result.