It's Evolution Not An Assumption
Species expand and adapt or they are eventually replaced by those that do.
I'll agree that intelligence may alter that trend, but not that we should assume it does or that ignoring that trend is the logical way to assess the probability of intelligent life. I believe we will not last a million years longer if we don't move out into space and our time may be considerably shorter if we don't. It's a simple matter of competition for ever more limited resources and fouling our nest.
Regards,
Mark