You’re right, they ran out of money. They never had a reliable funding source like the bin Laden network or rich Saudis.
The main reason for ISIS defeat is that they thought they could gather together as a subnational caliphate state and be recognized by other states and attract lots of Muslims.
They became a series of easy targets for our air power and artillery.
Also, their recruits were so radical and emotionally charged that they weren’t very trainable and refused to coordinate across a wide battlefield. Petty infighting and poor logistics kept them no better than a mob.
I think international policy should be to kill as many of these people as possible.
No prisoners.