I think it would be about 30% old Baldur's Gate fans, and 40% D&D 5e fans, with a lot of overlap between those two groups. I suspect around 50% (or more) of the game's buyers never played Baldur's Gate 1 or 2, and never played 5e D&D. But that's mostly just speculation and extrapolating from what I've seen from streamers and YouTubers playing the game.