Then how do you think most of the sorceresses in the past way before Adel's time died? Do you really think someone hunt them down and slew all of them?
Who's to say there aren't still plenty of sorceresses hiding around the world. The tutorial tells us that most sorceresses hide away, so it's impossible to tell how many really exist.

Of course, if they merely have extended lifespan, they'd still die of old age sooner or later anyway. Others may have been killed. Others died in accidents. Who knows? And remember that since sorceresses avoid spreading their powers too thin, the amount of sorceress in the world grows smaller with time. In other words, if a sorceress has a choice, she will give her powers to an already existing sorceress. So with time, the amount of sorceresses are reduced, even if one only dies every now and then.

But it is said that for a sorceress to die peacefully, she must pass her power to someone. It did somehow imply that sorceress pass their powers when near death. Edea's case is an accident.
This is a point of discussion; does the body force the sorceress to 'cough up' the powers before dying(ie. the body cannot let go of life while containing the powers) or is it merely that the sorceress does not wish to die while still a sorceress. The most recent information from Ultimania(still unconfirmed, this is another mystery we hope to solve soon) suggests that a sorceress wishes to pass on her powers because it's like passing on her genes, or legacy. So just like animals strive to pass on their genes, a sorceress does not wish to die without passing on her powers.