In the short view, that's often true. In the long view, it's not a one-way slide. Things eventually either totally fall apart or there is a backlash. The mechanism that causes the slide is that people never appreciate the role that moral behavior plays in making life pleasant for everyone. The mechanism for the backlash is that when things get unpleasant enough, people start to understand the value of morals and embrace them again. The only real question is how strong the backlash will be and, intuitively, I think the strength of the backlash tends to correspond to the depth of the moral decay that it's responding to. And, yes, a very strong moral backlash can be bad, too.