Rowling may never experience social comeuppance for her actions the way Bill Cosby faced legal jeopardy for his. Trans people are a relatively small subset of humanity, and unfortunately JK Rowling isn't the only person who thinks they shouldn't exist at all. (*ehem*Trump*ehem*) Trying to financially break someone like this by boycott is not going to make a difference to her, but that's not really the point. It will make a difference to the people who are being affected by her cruelty. It will make a difference to those who might think that my love of a pop culture character is greater than my love of real people with real lives.
So it's time to let this shirt go. Just as a lot of people got rid of some garish "Cosby sweaters" back in the 20 teens. And for much the same reason.