I think that people who think being openly GC won't affect her must live in a city bigger than Edinburgh, with more anonymity, or a small village with only a few people transitioning.
In Edinburgh there are enough TRAs for a movement, they have influence and power. There are also quite a lot of them in the quidditch teams, from what I've seen. You'd not want to be guessing any pronouns at a quidditch tournament.
The person who went for Julie Bindel wasn't a lone wolf, look at accounts of Edinburgh pride. JKR would be more aggressively targeted as they would actually feel betrayed by her, having first been fans. Edinburgh is small enough that people, not even fans, know her area, school choices etc.
She is extremely vulnerable in her real life, not her book sale life or her Twitter life, but her real life and family life. We need to remember that. She had to move house in the last couple of years to reduce security fears, this kind of celebrity is something we can't really understand.
Also, subtlety, getting people to think about the idea of being transabled, especially from the perspective of an amputee who was being fetishised, is a lot more than most people have done.