Generally, new stuff that is successful does not compete head-on, unless it's got some kind of "unfair advantage" like a big company behind it. In other words, it does something a bit better, but also differently. Rails was better organized and cleaner than PHP, but less bureaucratic than Java.

Chicago Boss can't compete with all the users and plugins and everything that Rails has, head on, so it's probably best used where it has an advantage on Rails: stuff like websockets where holding a lot of them open is a piece of cake for Erlang, but more of a resource hog for Rails (at least it used to be that way, has 4 improved things?).