>Starcraft 2 was over 6 million sales before its final expansion even came out.
GTA 6 preorders beat that in like a week and the game isn't even out and no gameplay footage was even show.
>That seems like the obvious player base for a Starcraft 3
A lot of that player based has aged out of the game or will stick to playing SC2 instead of spending 80-100$ on buying the same game over again. RTS have reached diminishing returns. There's not much more you can add to SC2 to improve it and convince people to spend on buying a new game. SC2 is feature complete.