The level of brand visibility, I think, is incomparable between Larian and BG (or D&D in general).
SteamSpy puts the BG3 sales estimate into 2-5 million range: https://steamspy.com/app/1086940 I think it's safe to approximate the number as 3 million. And that is just Steam alone - there's also GOG, and then there's Stadia. So yeah, I tend to agree with Veilburner that 6M might be a close estimate of the total sales.
I don't think they will be able to quintuple that number after release, like you say they did with DOS 2. The market simply isn't that big. I suspect they have already exhausted most of its capacity by now. They might sell about the same amount after release, but that's it.
However, in order to have these post-release sales, the game has to be both majestic and flawless. Considering the amount of copies already sold, the word about any issues will spread quickly. There are comparatively few players who are acquainted to the LTC mechanic (DOS fanbase), and the majority of users would expect a game they can call both "familiar" and "intuitive". Whether Larian will be able to deliver on these fronts... we'll have to wait and see I suppose.
