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?
I think you have totally misinterpreted the point made in TOS. NONE of those societies were true Utopias. They were fictional, often computer controlled [and imposed] prisons under the guise of Utopia. The Federation is nothing like that and I think you should perhaps rewatch a few of those episodes.
A true Utopia would not lead to stagnation. Hypothetically, with everyone able to pursue their own interests and better themselves unmolested, the potential for improvement and advancement is limitless.
Thank you; I don't need to rewatch them. I've seen them many times over the decades, I've read the Blish adaptations, and other articles and stories that explore those situations and themes.
WE don't see the Landru society as a utopia, but the people who live there do. Same as the people of Vaal; to them, everything is perfect. To us, it's stagnation.
Utopia implies a state of perfection. Kirk makes several speeches throughout the series where he points out that humans are not meant for this state of being; we need challenges.
As for the Federation being this wonderful, perfect society where everyone is happy and able to fulfill their highest potential... BS. Really. How many times did we see or hear of Federation planets (or portions of the society on some of these worlds) where things are not
this happy, fulfilled existence?
Earth in the 24th century seems idyllic, but how long can such a society be sustained? Sooner or later, everything
changes. The Federation may last for many centuries or even millennia, but it won't last forever. Nothing will.