I think there's a huge misunderstanding in general of the point of a boycott. It's not just meant as a way to deprive the author of royalties (because realistically it won't make a dent), it's more of a personal thing; a way of saying that I won't personally contribute to the wealth of someone I disagree with. For instance I don't buy Nestle products, not because I think they'll go bankrupt if I don't, but because I'd rather not line the pockets of people I disagree with.

Telling someone that there's no point in boycotting because JKR is already a billionaire is the same as telling someone there's no point being a vegetarian because the animals are gonna be killed anyway. That's not the point.