True immortality means living forever. Lasting forever is very hard even for a civilization or humanity.
What are things that put a species or civilization at risk of ending ?
1. Reduction in population size (50-70% or more reductions over 10 years).
Having a large and growing base of population can tolerate losing 100 million in a world war or tens of millions to smallpox.
2. Having fewer than 2500 mature adults puts an animal species on the endangered list.
Here is a graph of the history of the world jewish population.
It illustrates and how numbers and scale matter to survivability.
There is the fictional stories and movies about the Planet of the Apes. However, in reality there are only a total world population of about 500,000 great apes.
3. Having a geographic range of less than 5000 square kilometers.
If there was a civilization on a South Pacific Island. No matter how much they fortify the island, certain events like a Tsunami or Volcano could wipe them no matter how well they managed their resources.
We know there are events that occur every 200 million years or so such as biosphere endangering Asteroid strikes.
Category 8+ supervolcanoes erupt about every million years. A total of 47 eruptions of VEI 8 magnitude or above, ranging in age from Ordovician to Pleistocene, have been identified, of which 42 occurred in the past 36 million years.
There are cosmic events such as supernova, microscopic blackholes which could be put life in a solar system at risk.
Technologies that would permit an interstellar colonization wave should be developed by 2100
There are technologies which are likely to be developed well before 2100 which will enable humans to travel at 10-99% of the speed
of light and to have ships with high acceleration.
These technologies are:
Advanced molecular nanotechnology
Nuclear fusion space propulsion
Producing significant quantities of antimatter and harnessing it for propulsion
Advanced molecular nanotechnology and nuclear fusion will also enable large spaceships to be perpetually self sustaining. The ships will be able to gather all resources from asteroids and convert them for appropriate use.
People in spaceships with advanced nanotechnology and fusion would be able to detect a star going supernova and move away to escape any destructive blast.
Out current civilization is beyond disasters or wars that would wipe out any Island based civilization. A future nanotech-fusion space faring civilization would be beyond any planetary or solar system based disaster.
Self replicating colonization waves
A civilization using self replicating systems and traveling at 50% of the speed of light could send out a colonization wave that would colonize the entire galaxy within 200,000 years.
No natural physical danger is known that could wipe out a galactic civilization.
There is still the risk of advanced aliens or an genocidal war using advanced technology.
Here self replicating colonization waves beyond 50% of the speed of light should be safe if the speed of light limit was absolute.