[I could only find this novel as part of a collection of short stories by Asimov called “The Far Ends of Time and Earth.” The other stories were entertaining and clever, but not quite as thought-provoking as “The End of Eternity.” The book cover was a featureless old library edition solid blue, so I just picked my favorite Asimov picture obtained via a minute of Googling.]
Sometime around the year 2700, the secrets of time travel are discovered and Eternity is established. Eternity is a place existing somehow outside of normal Time; traveling through time is only possible when starting and ending from Eternity. So, the Eternals can’t travel back prior to ~2700 (easily), but they can go forward. Quickly a noble enterprise is established: Observers infiltrate various centuries and report their findings back to Computers (a person’s title), who calculate what Minimum Necessary Change is required to effect a maximum increase of security and stability and human happiness for the greatest number of generations. The changes, put into effect by Technicians, are small, butterfly-effect style stuff — like making someone late for a meeting — but with large effects — like if the tardiness to a meeting prevents some couple from falling in love and avoiding them conceiving the next Hitler, for instance.
After quite a while of this, these Changes have done great things like prevent world wars. Humanity is pretty content across many millenia. But as it turns out, by removing stressing stimuli the Eternals have also culled out mankind’s spirit of adventure and creativity. Far in the future, the Eternal-modified timeline leads humanity into a dead end: we stay back at home while other races colonize the galaxy, and the triumph of humanity, the Galactic Empire (same one from the Foundation series – all Asimov’s stories are related) never happens.
In other words, the Eternals optimized themselves into a local maxima. Shoulda taken that Discrete Optimization course!