Baldur's Gate 3 will be GOTY for the hardcore old-school CRPG fans that have been waiting for this game for what will have been 23 years. But it's part of a very niche genre, always has been. It's catering to a very specific crowd. Starfield, while having claims of being a hardcore RPG by Bethesda, is designed for the masses.
Most new players that will pick up BG3 are probably going to hate it because of the style of game that it is. It doesn't translate well to modern gamers as it owns its old school flare. It's going to be a GREAT game! No one that is a fan of Larian or Baldur's Gate is going to deny that. I've enjoyed it since it's been in Early Access. But, the game is going to be a modest commercial success at best due to its nicheness.
Saying Baldur's Gate 3 is somehow a threat to Starfield would be the equivalent of saying Pentiment was a threat to God of War: Ragnarok in November 
