That’s literally what plenty of laws do…

Laws tell you what to do. The inherent threat of fines and imprisonment compel you.

Depending on how the contracts were written (which happened before Rowling unveiled her transphobia) breaking from the films could result in litigation.

Often times this is correct. But the thing is if you make a big enough problem for a studio they won’t want to deal with you anymore. That’s a double edged sword though.

It’s a big hassle for them and honestly, do you think a cis actor dropping a nuke on the Fantastic Beasts franchise on our behalf would be noble or would it be a case of that actor co-opting our fight to score publicity points?

I honestly don’t care how selfish the motives are. Refusing to work for her is a good and noble thing.

Honestly if every trans-positive cis actor broke their contract because someone who would profit from the movie was transphobic, you wouldn’t have a single movie made for fifty years.

If you honestly think that the number of people who would be protesting in this scenario is so large as to derail the industry for even a tenth that long, then you must also accept that the studios would listen a lot quicker.

Try selling that to our society without giving the impression you’re trying to give everyone else in the world the same cultural misery you have.

Again, studios are greedy and would clearly choose the option that angers the least amount of people. See previous point.