Kevin Costner’s American accent in Robin Hood?

Exceptionally bad example using an English folk hero.

Yes, I get what the tradition is. But the tradition is equally silly. Substitution of one farce for another.

Just because that's the farce they always go with doesn't make it better, it just makes it what people expect. People objecting because the movie did something different are the same people crying about the cookie cutter tactics of the movie industry are ruining art.

The more formal speech helps set the tone and creates an immersive feeling that this isn’t taking place in the current time and place or vernacular.

Those are advantages of more formal speech. There are also advantages to using more contemporary speech - it can feel more accessible and relatable. There’s no one correct approach, it’s a matter of what tone best suits the film.