It's when they do silly things such as change the main character from a white man to a black woman, or make them gay, which has annoyed a lot of people, then they claim racism, misogyny, homophobia etc. when they get negative comments and it flops. Most people aren't bigoted. There have been successful films with non-white and non-straight main characters, but they were new and original, not a classic with the story changed to fit a progressive agenda and the actors chosen to tick boxes.
Agreed, those are shit. For instance, the Ghostbusters film with all-female cast was critically panned, because it was badly written and leaned very heavily on the female cast being "unique" rather than having a good story.
But this is bad storytelling, and I don't agree that it is all that common. Is it
more common? It seems so, I wouldn't say no... but it's certainly far from normal to have a plot that's just been turned around to fit diverse characters in it and flopped as a result. Hollywood isn't stupid and they want to make money from films (Ghostbusters 2016 lost money).
I'd say the BBC are currently experimenting with a new Doctor in Dr Who, they tried Jodie Foster which I thought was a bit of a damp squib and unfortunately a letdown for a female Doctor. She had terrible scripts, she's historically been a very good actor so it could have worked if the writing and direction was done properly. I'm looking forward to seeing how Ncuti Gatwa plays the 15th, I will approach it with an open mind, as they have Russell T Davies back to run the show and he knows how to write a good Who.
For a film with a diverse cast that was very well received look at Black Panther, I think there are two main white characters in the whole film, Martin Freeman being one of them. The sequel was okay too but not as good as the original, the villain was boring.