I agree 'war on terror' is idiotic. Terrorism is a law and order issue. It's an illegal act. Yes sometimes the perpetrators have state sponsors, but that doesn't really change anything about the nature of the crime, though it does actually open up additional means to fight it by applying military, economic or political pressure on that state.

In the case of e.g. Iran they've basically been brought to the negotiating table by economic and political pressure. They're still far from clean, but at least they're tractable. IS on the other hand is functionally a state but politically and economically intractable. We and several Middle Eastern nations are at war with them. However their terrorist activities carried out outside their territory are still criminal activities best dealt with as law and order issues.

Terrorist acts are crimes. Very rarely does the definition of a crime include specific criteria on the nature of the perpetrator. In other words the crime committed only changes based on who committed it in pretty specific ways. For example there's a crime and there's conspiracy to commit the crime. Unless the definition of terrorism explicitly defines it as a crime committed by groups, I see no reason to exclude acts committed by individuals acting alone.