But to be honest, Starfield is not really something I see as a competitor for BG3, they are too different. It feels to me like saying Warcraft 2 is a big competitor for Heroes of Might and Magic, since they are both strategy games. Way too different, scratch a different itch.
It is absolutely a competitor to BG3 in the sense that both are games that are basically black holes with people's free time and it's going to be massively popular among a very similar target audience.
Larian did the wise thing moving their PC release date out of the way because they would have been slaughtered otherwise. And to be clear, it's not even a matter of which game will be more popular (spoiler: it's Starfield). They would cannibalize each other regardless of which comes on top (still Starfield, by the way).
Please note that I say this as someone who A) is far more interested in BG3 B) was never really particularly fond of ANY previous BGS game (in fact, I consider an anomaly the fact that so much about Starfield is appealing to me, from concept to aesthetic).
I think you might be underestimating bg3 right now.
Asmongold had a video 13 days ago that has 888k views right now over the AAA studios freaking out over how good bg3 will be, his last video on starfield was a month ago and only has 681k* views. Starfield is #11 on steam, BG 3 is #3.
Diablo bros are hungry for a new game, especially while they wait for their new patch. And they are probably going fantasy
Meanwhile Starfield has had really all it's media hype evaporated between Armored Core 6 Diablo and Bg3 controlling the cycle.
Armored core 6 is another big issue for Bethesda as it competes with the gritty space asthetic and is within weeks of Starfield.
Between Star Citizen Bg3 and Armored Core 6 Toddy should be afraid.