My understanding is that JKR doesn’t even get royalties; that she took a lump sum license payment. In other words, she already got her money, regardless of whether the game is successful

A boycott, if successful, serves to tell other companies not to do business with JKR in the future. That’s a perfectly reasonable thing to to want.

Where we get into an issue is where people make such a boycott a moral imperative; like telling people they are bad people because they chose to enjoy a video game that a shit person profited from. Personally, I’ve chosen to skip this game because, well: fuck that TERF — but I certainly don’t think you’re evil or anti-trans because you arrived at a different conclusion.