The defect in this reasoning is the use of the singular word "it" in the last sentence. This is not a collective decision where all intelligent life on a planet monolithically decides whether to ever migrate. Or is it? Here's the key point: The ONLY way for a planet to avoid becoming a seed for space colonization is for ALL intelligent life on that planet to decide not to migrate for all time. How likely is that, especially since not migrating is suicidal?Yes, of course that is true; replication is an essential part of a Darwinian scheme. We don't know, however, if Darwinian evolution continues forever. It might be the case that Darwin takes us to a certain point, and something else takes over. Look at the population declines that are occurring in many developed countries. Something non-Darwinian seems to be going on already. If mankind's pinnacle is a superintelligent AGI, why exactly would it care to colonize the universe?
Ironically there is a banner ad for the Lifeboat Foundation on my screen. Even now there is a minority that advocates diversification of human civilization into space as a matter of civilization survival. For immortalists, it's a matter of long-term personal survival as well. If technological progress and economic growth continues, eventually the wealth of a minority that wants to migrate will rise to meet the declining cost of actually doing it.
As long as there is diversity of thought, and as long as there is technological advancement, there will eventually be space migration. Diversity of thought and advancing technology seem hard to avoid without some kind of totalitarian collapse.