There's also some assumption here that civilisations either collpase or conquer the galaxy, but that ignores another possibility - that civilisations might quickly reach a plateau technologically and in terms of size.
The reasons this could be the case is that civilisations must always solve their problems of growth and sustainability long before they have the technology to move beyond their home planet, and once they have done so, there ceases to be any imperative toward off-world expansion, and without ever increasing economies of scale, technological developments taper off.
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.