The fall of the Imperium was there for all to see from the very first adventures and supplements.
On the frontier, the Spinward Marches, there was an awful lot of unrest and dissent, not to mention blatant Imperial corruption and oppression.
I interpret that as a case of "not being blinded by dreams of utopia" more than anything else.
Even when a polity is prosperous and "things are going well" under benevolent governance, there will be malcontents lurking about eager to stir up trouble. Interested Parties™ will always be trying to advance their own parochial self-(ish) interests over the whole. It was an acknowledgement that the Third Imperium was not a "perfected society" that was all harmony and unity.
Besides, those pockets of unrest and dissent, as you cite, along with corruption and oppression, as you cite ... created all kinds of opportunities for adventure seeds and adventure hooks.
Then in S:3 we are told the Imperium is:
"the third human empire to control this area, the oldest, and the strongest. Nevertheless, it is under strong pressure from its neighboring interstellar governments, and does not have the strength nor the power which it once had."
Which then begs the question ... where are these neighboring interstellar governments?
Look at the Spinward Marches sector map.
The Third Imperium "owns"
HALF of the map outright (Regina, Lanth, Lunion, Glisten, Aramis, Rhylanor, Mora, Trin's Veil).
In the other half of the map, the Third Imperium is dominant in 3 of the 8 subsectors (Five Sisters, Jewell, Vilis).
It's difficult to look at that map and think that the Third Imperium, as a polity, is "seriously threatened" by the Darrians (allied protectorate of the Third Imperium), the Sword Worlds (aggressive, but limited to TL=11-12 and having only ONE type A starport to build starships at in LBB S3) or even the Zhodani ... who hold an "oddly shaped corner of the map" that combined amounts to a single subsector's worth of worlds that have a relatively inconvenient set of trade codes (so they're not even as powerful, collectively, as the Sword Worlds).
The Vargr
aren't even on the map of the Spinward Marches.
The Aslan are over a sector away to rimward.
With a technological advantage (TL=F), the only "real threat" to the Third Imperium in the Spinward Marches (militarily) was ossified incompetence, complacency and corruption eating away at the muscles and bones of "power" (both hard and soft) from WITHIN the Third Imperium, rather than any kind of external pressure from neighboring interstellar governments imposed from without.
The thing that made the Third Imperium boring was the Fifth Frontier War, it was boring to have three years of TAS News with nothing but battle reports. Prior to the war the TAS News items were full of adventure seeds and setting details.
Agreed.
The way that the Fifth Frontier War "blew up out of nowhere" felt like more of a "forced narrative" rather than something that developed organically with "plenty of warning" for the franchise. It was a "shocking development" that felt like it came out of nowhere ... much like the assassination of Strephon.
Carefully build the sand castle ... then STOMP ALL OVER IT before the tide comes in to claim it.
Yes, yes, I know ... there were hints and nuggets and tips and tidbits getting dropped here, there and everywhere in the adventure books prior to the FFW kicking off, but those were all disparate unconnected threads that weren't laying the ground work for a three-way sneak attack into the Jewell, Vilis and Regina subsectors. If you hadn't been buying EVERY book and EVERY JTAS supplement, you had no basis for understanding where the FFW came from ... let alone what events might have precipitated or foreshadowed it.
Which made the whole FFW "event" feel like a
GOTCHA! rather than something that the setting had been carefully and methodically laying the groundwork for. It was like one of the old mystery novel tropes where the detective reveals withheld clues and information at the end, including details about characters previously unknown until the last 5 pages of the book (in other words, the author "cheated" their audience and readers).
Then the same stunt got pulled AGAIN with the assassination of Strephon ... for shock value.
It was almost like the people in editorial control of Traveller were
taunting the Referees and Players ...
"We know something YOU DON'T KNOW!"
And it happened AGAIN with the Empress Wave nonsense.
And the Virus nonsense.
Just ROFLstomp after ROFLstomp after ROFLstomp ... until the OTU was barely recognizable anymore.