That respect of Banks always felt a bit handwavey as to the specifics. (I.e. good/freedom triumphs over evil/tyranny, because it's a superior philosophy)
At galactic-scale, across civilization timespans, it's not as apparent why that should hold true.
Would have hoped that Banks, had he lived longer, would have delved into this in detail.
Granted, Vinge takes a similar approach, constructing his big bad from an obviously-not-equivalent antagonist, sidestepping the direct comparison.
The closest I got from either of them was that they posited that civilizations that tolerate and encourage diversity and individual autonomy persist for longer, are thus older, and that older counts for a lot at galactic scale.
^ Note: I'm asking more about the Idiran Empire than an OCP.