Stopping the master from getting out was a good threat to have. Its not the end of the world if he does. Its just another really powerful vamp out in the world. Buffy verse has lots of them. On the other hand, acathla who will literally suck the world into hell, glory tearing open all dimensions so they blend together, the first evil tearing open the hellmouth and unleashing an army of prehistoric vamps, etc etc etc. All way over the top. A fanfic I read described it like this. "All of existence is like a high rise building. Hell is at the basement, and its constantly trying to climb up a level. Every time it does its one level closer to reaching heaven and we are there stomping on its fingers as it tries to pull itself up." Which is a TERRIBLE universe! It basically is saying eventually, hell WILL win, because there is no mention of taking back levels. Eventually, hell will inch its way up level by level and reach the top.