Reactions like this gave us Black Panther and Wonder Woman. Also I am unclear how this kills creativity. If they cast a trans actor in the role, creativity is killed? How?
Context matters, and that is what is being ignored. You say it is a dumb question, but it hits at the heart of this. 60 years ago, Micky Rooney could be a racist caricature in an American film. Progress is slow, and never perfect (Ben Affleck played an Latino man 5 years). But in 60 years, what are the chances people look back on us, the same way people look back on black face? Asking why did we think it was okay? Because the same excused being used now, and not different from what we got when they would cast white actors in black, Asian and Latino roles. There aren't enough actors, there aren't enough stars of that race, the movies won't make money or get made, etc.
Also, there is one huge, obvious flaw comparing gay or straight actors playing the opposite. Namely, they will cast a gay actor as a straight character, and vice versa. How often do trans actors get ask to play non-trans roles?