Nothing lasts forever. Everything breaks down eventually. It's just the nature of existence. In Star Trek, the Federation has been on a steady upswing of power and expansion. It is unquestionably the most powerful of the major Alpha Quadrant empires. However, compared to say the Romulans or the Klingons, it's still fairly young even by the 24th century. I'm wondering though, how big can or should it get and how long would you expect something like it to last? Within the on screen canon it's still around in the 31st century (or atleast there is a monument to it on Earth). I think most of us just assume that eventually it will expand and simply assimilate the entire galaxy and effectively become the Galactic Republic from Star Wars....but should it? Maybe it should become as absurdly large as the Systems Commonwealth from Andromeda and encompass several galaxies. Star Trek has always been about presenting a positive vision of the future...thus does not consider that things, no matter how just or well crafted, fall apart. Is that a story you would ever want to see told? Related to this...if anyone has ever read Christopher's wonderful Trek novel, The Buried Age, there is some indication that there is a distant future where humans and the Federation were not clearly recognizable or even remembered.