How do both failed? Charmed centered around 3 sisters, they killed off one of the sisters and brought in an entirely new one, that's not a small thing, it's kind of huge. Prues character was just as big in season 3 as it was in past seasons. And Buffy wasn't cancelled 2 seasons after Angel was spun-off. Buffy spent 4 seasons Angel-less and it wasn't even cancelled, it ended on its own merit. So both of them are perfectly good examples of shows getting rid of major characters and still having life afterwards.