Science Fiction has a number of tropes, and Golden Age CT only ever hit a subset of them.
CT's brush with interstellar war (the FFW) was, when the shooting stopped, just that. A brush. Neither civilization nor any empire was hanging in the balance. The social changes that accompany extensive war did not take place. A year later it was as if it hadn't happened at all.
The "righteous" wars of Imperial expansion were centuries gone, as were the years ruled by the Fleet Emperors. The "rebellious province" had been a century before and was on the other side of the Imperium.
Without hopping into a historical frame or travelling for years just to find the frontier, the age of exploration was done. The Imperium was hemmed in on all sides.
Where does a setting go in such a case? Especially a setting as big as the Third Imperium, that has been stable, aside from those aforementioned border scuffles, for centuries. If you are trying to sell an RPG where a large number of the players are already playing gun-toting scofflaws, mercenaries, or "off the books" agents, the answer is pretty obvious. You go to war. Real war. Empires-in-the-Balance war. You introduce larger-than-life personalities who will drive larger-than-PC events.
RPGs in general suffer from static settings. Companies lavish vast effort on snapshots of large and detailed settings. Lengthy (often to the point of silly) histories are concocted to explain the current state of affairs. Rulers, movers, and shakers are all detailed. Then the setting... just... stops.
A few SF settings are noteworthy for avoiding this, and Traveller is the oldest of them, but the Third Imperium was showing signs of having that same problem. "We printed it this way, we can't change it now." Of the settings with the most movement (the other two that come to mind being Battletech and Shadowrun), Traveller was the only one that could get away with stasis, but even so there is only so long you can claim to move the timeline along on news stories of technology advancements, luxury liner launches, and "man bites dog".
And so we got the Rebellion. Like it or not, time marches on, and the Golden Age fans have a century or more to play around in. The thousand year empire got eleven hundred years in total, but now the Emperor is dead, and it is time to let slip the dogs of war and make some new history.