Linear And Non-Linear Future Histories

Isaac Asimov's Foundation Trilogy begins untold thousands or tens of thousands of years in our future. Several later written volumes recount intervening history. After an incoherent time travel scenario, mostly set in other timelines, there is an interplanetary robotic economy, then two phases of extrasolar colonization followed by the growth of the Trantorian Empire that becomes the Galactic Empire.

The Trilogy opens as the twelve thousand year old Empire begins its terminal decline. Hari Seldon's psychohistorical Plan will reduce the interregnum between the First and Second Empires from a predicted thirty thousand years to a mere thousand. The Trilogy covers only the first four centuries of the interregnum. Two subsequent novels add one more century.

Although the series is set so far in our future, human lifespans have not been extended. (By contrast, after the opening volume of James Blish's Cities In Flight future history, the reader does not notice that centuries are elapsing because the anti-agathics preserve a small number of interstellar travelers until the end of the universe - which, however, is brought unexpectedly close to the present for narrative convenience.) Thus, none of Asimov's characters survives for more than a century.

Despite the absence of continuing characters, the Trilogy remains an entirely linear narrative. The Galactic Encyclopedia Foundation on the planet Terminus becomes a Mayoralty, with Traders and Merchant Princes, successively interacting with:

imperial provinces that become independent kingdoms;
the weakened Empire;
the Mule who upsets Seldon's plan;
the hidden Second Foundation that restores the Plan;
the planetary collective consciousness called Gaia that secretly manipulates the Second Foundation;
the immortal telepathic robot, Daneel Olivaw, who is ultimately behind both Seldon's Plan and Gaia and even indirectly the Mule because the latter turns out to have been a rebel Gaian, not after all an individual mutant.

The subsequent novels diverge from the original Plan first by introducing Gaia and secondly by reintroducing Daneel from the first extrasolar colonization period. Despite this divergence in content, the structure remains chronologically linear with each installment a direct sequel to the preceding one. An indefinite number of otherwise independent stories could have been set, for example, in the Traders period but Asimov did not go down that route. Instead, each new installment had to advance the timeline and progress the Plan, although ultimately Daneel's, not Seldon's.

Comments:
extremely far fetched;
more about implausible social manipulators than about credible social developments;
differing from the alternative future history model created by Robert Heinlein and followed by Poul Anderson and Larry Niven.

The Heinlein Model:
several successive historical periods with a number of otherwise independent stories set in each period;
transitions between periods explained either by pivotal stories or by background information in later stories.

Thus:
Heinlein's "If This Goes On -" informs us that the Prophets had seized power and describes their overthrow;
Anderson's "Cold Victory" informs us that the Humanists had seized power and describes their overthrow.

However:
Heinlein devotes several stories to the daily lives of ordinary people on the Moon in the pre-Prophetic period, then two to the changed social conditions in the post-Prophetic period;
to a lesser extent, Anderson shows us daily life on Earth and a colonized asteroid in the pre-Humanist period.

My point, as ever, is that I prefer Anderson's several future histories to Asimov's single future history! Even Anderson's earliest, Psychotechnic, history proves to be more substantial than expected when reread with sufficient attention.