Obviously if someone is genuinely leaving then they should be given a chance to say goodbye. Unfortunately we have a lot of people who post goodbye threads with absolutely no intention of going anywhere, just to get attention or to get support in some petty battle with another poster.
One solution would be not to ban goodbye threads, but to ban the people who post them. Temporarily, of course. Sometimes people genuinely intend to leave, but then later reconsider. An automatic one week ban would be no problem for those people, but would make the people who post fake goodbyes reconsider.