For me, the Federation should last indefinitely. It is a symbol of what Trek is about and should always be a part of the show.
As much as you say 'everything falls apart' we have never seen an entity like the Federation before. Sure it isn't perfect, but it is vastly better than anything else around it: I find it perfectly believable that it would absorb most of the galaxy given enough time.
I simply dont see it falling apart and I really dont think it should: it is a part of the Star Trek ethos and I really do not like those ideas of a Federation in turmoil or some post-Federation Trek series....Trek is supposed to be positive and the Federation is the absolute representation of that.
So after the Federation becomes this happy-perfect place, what then? Utopia is boring. Utopia leads to stagnation. How many times was that point made in TOS?
It is true that every complex human society in history has eventually collapsed... but then, it is also true that every complex human society in history was, in some manner or other, built upon the principles of inequality and oppression.
Personally, I think the Federation -- a society that has truly eliminated oppression and inequality on a scale no human society in real life ever has -- could plausibly last as long as humanoid life in the Milky Way Galaxy lasts.
And, of course, it's just as possible that the Federation may eventually peacefully persuade every other society in the galaxy to join it.... or that it may evolve into something else, something new. Maybe the Federation will encounter another interstellar liberal democracy and join with it to create a newer, larger interstellar democratic union, of which the Federation merely becomes a constituent part. Who knows?
Unless human nature changes at its most fundamental level, I think this is extremely optimistic.
This is a meaningless argument. "Human nature" encompasses everything from Gandhi to Hitler. Nothing about human nature is universal or inevitable.
Besides, Humans aren't the only species in the Federation.
The lessons of history tell us that so far, every political entity falls or fundamentally changes at some point.
And as I pointed out, there has never been a true liberal democracy in history until the last 50-100 years. Liberal democracy is an experiment that we're undergoing now; history has no precedent for a truly egalitarian democracy.
Which is not to say that it WILL last in real life, of course. Perhaps the human impulse for oppression and hierarchy will overcome the human impulse for liberty. I don't know. But in the world of Star Trek
, which is after all based on the presumption that Humanity's political culture has
fundamentally changed as a result of World War III and First Contact, and wherein Humans are only one of many species united in equal partnership under the Federation banner? I think the Federation may last indefinitely, in some form or another.