Correct me if I'm wrong, but I thought it was not just enough to have a phrase enter the public vernacular, but that it would have to be in the vernacular in such a way that its no longer directly associated with the product that was trademarked.

I like the general idea though. It seems that if the general public no longer associate a work of art with the artist or company that holds the IP, it should go into public domain.

In this case, Happy Birthday probably never would have been popular in the first place had the copyright been aggressively enforced in the 1920's. In my opinion, the lack of defense of broad copyright infringement from 1920-1980 should be grounds for copyright invalidation.

Generally copyright has lost its original intent of providing value and protection to the original creator, while encouraging people to create something that will eventually become part of the public domain.