My own theory is Darwinian inevitability
>Given enough time, even simple, expanding, problem-solving organisms will breach every barrier—including space.

Either through adaptation like a simple Tardigrade or continually problem solving. A great barrier can't exist because we've already reached past the point of expansion. Even if the species were to be killed off today, another one with enough time will eventually reach the same conclusion and try again. The inevitability of it means that there is no way to stop any species that is a carbon-based lifeform from expanding. A colony of ants or microbes can “solve” space just through iterative evolution.

Global catastrophes don’t end evolutionary progress. They delay it. Life begins again and keeps climbing. And life has the ability to be essentially immortal under the right circumstances.
The only issue might be that such simple lifeforms don't emit radio waves or build Dyson spheres.
They just spread. Slow as molasses.