Could the seeds of the collapse of the Federation have begun after the events of Nemesis?
Picard said a line in the movie that made me wonder when he said they had various enemies trying to take bites at it so I wonder if the cracks were starting to form then after the events of that movie and into the future. So yes the Burn happens but the seeds were planted long before that.
I think the opposite is true. I think, from the perspective of the 32nd century, everything we've seen in the 24th century is the dawn years of the Federation, and the Dominion War was the Federation's coming out party as a galactic-scale power.
I am reminded of the line in Q Who? when Q tells picard that the Federation will begin moving into areas of space in coming years with dangers that make the Romulans and the Federation look like nothing. And True enough, within a few years of that encounter, the Federation encounters the Borg Collective, which has controlled a large portion of the Delta Quadrant for millennia, and the Dominion, which controls a 2000 year old empire in the Gamma Quadrant several times the size of the Federation. And that's just what we know of. There could easily be more Borg and Dominion-scale dangers out there.
By 2379, the Federation had triumphed over both. The Dominion's expedition to the Alpha Quadrant was soundly defeated by the Federation Alliance. A rematch would be in a couple hundred years, at the Dominion's reckoning of when they expected to find Odo. The Borg were badly damaged, maybe destroyed, by the actions of Voyager in 'Endgame". The Borg, spreading like a virus across the Delta Quadrant, were by far it's greatest power, and probably the greatest caporeal power in the galaxy.
And then, let's recall what Sloan said: at the end of the Dominion War, the two remaining powers in the Quadrant would be the Federation and the Romulans. That was before the Supernova of 2387. By 2399, there is no more Romulan Star Empire.
This leaves the Federation with a more or less wide open field, with one crucial advantage up their sleeve: the technology that Voyager brought home from the Delta Quadrant. Locally, they have no real rivals.
I wrote this before but I think the Trek books pretty much nailed it: Voyager's most important legacy is bringing Quantum Slipstream technology to the Alpha Quadrant - a next gen galactic-scale propulsion technology that is compatible with federation technology. From a historical standpoint, propulsion had been at a relative standstill. Between Archer and Kirk's time, starships became hundreds of times faster. But between Kirk and Janeway, they got to be only twice as fast. True, compared to the late 23rd century, ships in the late 24th were far more reliable, efficient, capable powerful. And true, the increase in propulsion allowed the Federation to coalescence into a more formal interstellar state over the 24th century compared to a looser alliance of the 23rd. But Warp 9.9 could only take ships so much further within a human life time. It is a late 24th century problem of why the Warp 5 engine was necessary in the first place, 200 years prior - because Warp 1 and Warp 2 weren't enough to do more than truly local activities. And to a degree, the Federation was truly stuck within a "local" sphere bounded by the Romulans, Klingons, Cardassians, Breen, Tholians and Ferengi.
Quantum Slipstream changes that. It opens the entire galaxy up. Like in the books, as soon as ships are built around both warp and quantum slipstream, then there the Federation can send missions across the galaxy on the scale of months.
I think that's the foundation for the 350 member Federation before the burn - it went from being a local space thing to a truly galactic thing, and Quantum Slipstream or propulsion technology like it opened it up.
If we want to divide the era of the Federation into periods from what we know, this is what I think it looks like:
2151-2399 - the "Dawn" years. The Federation is formed, develops ever better warp drive and becomes the dominant power in local space through a series of treaties, wars and luck (decline of Cardassian, Romulan and Klingon Empires). The period ends with the Federation spanning over 8000 light years, and emerging victorious against it's two most dangerous adversaries from the Gamma and Delta Quadrants, and finally coming across a technology to succeed warp over great distances and allows true galactic-scale exploration.
2400 - 2700 - the "Age of Expansion". The Federation utilizes new technology to explore ever further from its origin point in ever more capable starships (including the Universe class Enterprise-J of the 26th century). The entire galaxy is open to it. Nearly all of its former adversaries in local space become members of it. The Federation faces few conflicts during this era and grows to nearly 300 members the entire galaxy ("planets" being a loose word, as the entire Klingon Empire = 1 "planet"). By 2700 it's far and away the greatest power in the galaxy.
2800-3000 - the "Temporal Age". The Federation is still expanding across the galaxy, but at a slower rate. It encounters entrenched powers, old adversaries left alone (the Dominion?) who are not amenable to it. In parallel, it starts to explore time with a fleet of time ships. The other powers of the galaxy at the time, unable to defeat the Federation militarily, decide to attack it across its time line to uncreated it. As a result the Temporal Cold War begins, This leads to the Temporal War, which is by far the largest conflict the Federation ever fought. The scale of the devastation across time and space is vast, only equaled by the resources thrown at it. The Federation prevails and the Temporal Accords are set, but the war broke nearly 700 years of peaceful expansion and security.
3000-3188 - "The Age of Decline". The Federation never full recovers from the Temporal War, nor it's own extraordinary long term growth. It eventually begins to fissure before the burn, and then fracture after it.
3199 - "The Age of the Second Federation". The Discovery crew and the Federation begin their long term rebuild of the Federation, world by world.
So no. I don't think the Nemesis era set the stage for the decline. If anything, I think the Dominion War / vanquishing of the Borg / fall of the Romulan Empire and decline of the Klingon Empire represent a breakout moment for the Federation to become the type of power that, by the 26 century, could build something like the Enterprise-J and by the 29th century something like the USS Relativity. I think the Federation's source of decline is ultimately in what it took to win the Temporal War truly cost it everything, on top of the age of problem of being spread over far too much territory with all the administrative complexity that entails.
Really though I would just like the show to to throw a few lines to etch some things in stone next season: something highlighting how the wars of the late 24th century cleared the road for what the Federation would become, and something to highlight that Voyager brought home advanced technology that changed everything for the Federation.
