A notion I've been playing with on and off is that while yes, that is the intention, eventually competition with the Federation will arise - another union of worlds with similar benefits to offer to those of the Federation, but perhaps with a less rigid set of requirements for admission (for instance, more flexibility in the government types allowed in). I think we might be seeing the beginning of that with
the Typhon Pact. Sort of a cold war situation. At some point, the Federation will find itself embarrassed by being caught out very publicly failing to adhere to its own ideals (think of, for instance, the events of "Insurrection"), and this rival will capitalize on it, and some potential new member worlds will look to them, instead. Some smart potential member worlds might even play the two large alliances off of each other, allowing them to effectively bid for them joining - sort of like the events of "
How Much For Just The Planet?". And some of these negotiations might even serve to further democratize the Typhon Pact (on the scale of member worlds, not necessarily individual citizens) and make them increasingly appealing to potential members.