[Ferret7446]

There are two cycles that govern human social change: the generational ~10 year cycle and the lifetime ~80 year cycle. Each generation adopts new values, and once a generation dies their values disappear. Tectonic movement is so slow that I think reality will be much more boring than your scifi story. The hundred thousand years where the two regions are "close enough" is more than enough time for the two peoples to both integrate and experience conflict hundreds of times over.

[epivosism]

True. Although most science fiction which would take place over such a long timespan might involve methods of locking things down more firmly, and putting at least part of the world into a kind of stasis. This is almost necessary to explain how humanity could still be at a similar tech level to us now yet still be around for such a time period.

[shiroiushi]

This will change if technology is developed which greatly extends human lifespans. Such technology is going to become necessary soon because of plummeting birth rates.