Distant future

See also: distant past

Common:
* Post-apocalypse

Even the "short" period of a millennium is enough for this.

Often employed to allow going overboard in the science fiction.

Relatively low technological improvement over modern times would indicate stagnation of technology or possibly even a technological dead end.

Examples of why going too far into the future is unnecessary or purely fantastic...
Unpredictable events: (natural disasters)
* Super volcano
* Large meteorite impacts
Predictable short term events: (most involve humans destroying themselves)
* Nuclear holocaust
* Rampant technology (e.g. nanotech, biotech, and many others)
* Overpopulation: mass famine; which may cause ecological collapse: oxygen and other vital resource depletion
* Unforeseen consequences of technological singularity
Predictable long term events: (age of the universe)
* At around 4-6 billion years from now, the Sun has expanded to such size to engulf Earth, it's total size exceeding that of Earth's orbit. Earth itself likely baked to death or fallen out of orbit long before this. After which Sun would die out, so colonizing the farther away planets of the solar system would only be temporary solution.
... and variety of other much more obscure events or longer term, such as Earth reaching the galactic core.