However, I can't say I listen to or follow Billie Eilish - I only found out about her coming out from reading this, in fact - but even as someone who's just mildly aware of her, I will say I've never assumed her to be straight. My gaydar's probably better tuned than the average person's but even so, I feel like her being some flavour of queer was fairly obvious and I wouldn't be at all surprised if that's influenced / shaped the kind of audience she has.
Unfortunately, I suspect an artist who's less obviously queer would lose a larger following for coming out. But yes, Billie Eilish only losing 100K followers is really encouraging still!