It is possible that most wizards/witches die not of old age, but simply because being a wizard is such a HUGE occupational hazard. If you use magic every day, you can make a fatal mistake eventually; especially if your magical power keeps increasing with age, whereas your memory / ability to concentrate goes down. And there are everyday spells that can easily kill you if you get them wrong, Apparition comes to mind. Yet it is possible that death in a magic accident is so common that it isn't even viewed as big tragedy. You'd have to be either extremely good (Dumbledore), or very careful (Moody, McGonagall), or both to survive to an old age.
I wonder why they haven't simply renamed the course. If it was called "Battle Magic", would V's curse still apply? What if it was something completely new, like "Sunshine Course"? What if the name was changed to, say, "Transfiguration"? Let the students have McGonagall's Transfiguration and Quirell's "Transfiguration". Students might get a little confused on their first week, but the benefit of having the actual education outweighs the cost... (and btw it's legal to have 2 professors of the same subject -- in canon HP they had Trelawny and Firenze both teaching Divination; not to mention that the headmaster is free to create new positions, like he did with The Keeper of Keys and Grounds at Hogwarts).