I think that you pegged it right with the aurors being somewhat like the FBI, but I think more specificaly they may be a mix between ghe FBI and a SWAT team. They don't handle the mundane stuff . . . exploding toilets etc. but they take on the dark wizards, and though ones like Voldemort and Grindewald only come around every 50 years or so, there will always be wanna-be's causing trouble. However, I think that after Voldemort Harry may find himself bored dealing with such inept dark wizards, then he may decide to teach.