It was vital that she have all the Horcruxes destroyed. Without that, there was little point in doing anything else. Sure, she could leave it to the Dumbledore brothers, but it was no sure thing and even after that Voldemort would need to die. It was too easy to forget that no one had known of the Horcruxes because no one had ever gotten close enough to try killing him. Even without Horcruxes in the picture he was a formidable wizard, and certainly difficult to kill.