[eschaton]

It also could be that interstellar travel is possible but never inexpensive enough to be casual or useful for ever-expanding colonization. Or it could be that civilizations stabilize before the point where ever-expanding colonization becomes attractive.

As an example of the latter, look at birth rates in different societies on Earth: Almost universally, they decline to replacement level once they hit a certain level of per-capita wealth.

It’s very likely that a society that achieves interstellar travel will do so after it achieves the ability to provide the highest standard of living for all of its members indefinitely using just the resources of its local system. This already describes Earth; the reasons we don’t do this are ideological, not based on any inherent constraints, while interstellar travel isn’t in our grasp yet and is likely to be extraordinarily costly.

Such a society wouldn’t face any pressure to grow, so any colonization would itself likely be ideological—“We don’t want to do things Surak’s way, let’s pull up stakes and find a world where we can live the way we want!”—or as a contingency/hedge against large-scale existential risk. Neither demand colonizing even a small fraction of a galaxy, assuming habitable worlds are even remotely plentiful near and reachable from the origin world.