There was a time at Hogwarts when the Defense Against the Dark Arts lessons were inconsistent and confusing because the professor in charge changed every year, or died, or ended up in St Mungo's, or was fired for being a werewolf. It was a dark time, certainly, but since then the world had changed. Voldemort’s attempts to return to power were a distant memory that no one willingly thought about, and when he died, the curse on the Defense Against the Dark Arts chair disappeared with him.