I think role of subliming was to show Culture not as pinnacle of existence but an advanced and powerful niche civilization.
In Excession main gimmick is an object (possibly unrelated to subliming) that immediately puts on alert all civilizations at Culture level of capability (and below) because it's something new in the universe, something that could help bump out of local technological minimum that the Culture currently occupies and exceed its potential in a different way than subliming, that is well known, but mostly one way road out (and very out).
In Excession main gimmick is an object (possibly unrelated to subliming) that immediately puts on alert all civilizations at Culture level of capability (and below) because it's something new in the universe, something that could help bump out of local technological minimum that the Culture currently occupies and exceed its potential in a different way than subliming, that is well known, but mostly one way road out (and very out).