I feel a need to add an important extra wrinkle. The front line of progress and growth in all things is driven by those who are pursuing progress and growth. If an upload cult arises in a planet, and 90% of the populace retreats into simulations, the remaining 10% are still around and we be selected to be exactly those people most invested in the material universe; within a few years (not millions of years, certainly) there’s a good chance they’ll be considering expansion. Or, consider how often new civilizations are formed by small populations of refugees from intolerably oppressive larger civilizations. Something like a war, extinction event or ecological difficult is similarly likely to prompt a desire for expansion as it is a total collapse.

Science fiction is full of sentences of the form, “The Klaatu were an alien race who colonized the galaxy a million years ago but have since moved on.” I always roll my eyes at this. Almost no matter what the Klaatu are or what they care about, there would be some group of them remaining. And the only Klaatu we would see would be the descendants of the ones who stayed. Giant galactic civilizations made of individuals are unlikely to make coherent collective decisions with no dissenters for the same reason our single planet (with much less serious communications obstacles) never makes unanimous collective decisions.