Anne made it very clear back in 2004 (after she missed out on the chance to play Christine in The Phantom of the Opera in favor of her Disney contract) she was ready for dark and gritty characters. And it shows what an accomplished performer she is. Havoc, Becoming Jane, Rachel Getting Married, Love and Other Drugs, The Dark Knight Rises, Les Miserables the film, and Interstellar were excellent choices in letting her showcase what a chameleon she is.
She is more then just Mia. Now this doesn't mean she is ungrateful for Disney giving her a break-through (Same with Amy Adams as Giselle in Enchanted) but there is so much more to explore when picking roles.
I, personally, really like the direction the second film took Mia, it might mark the first Disney has ever given a princess a happy ending that didn't revolve around a man and that a young woman doesn't need a man to begin her life is a message that needs to be instilled in young girls. So as much as I'd miss Ms. Hathaway in the role I'd be interested in seeing what they can do with the character next.
According to the article provided, Julie Andrews has stated interest in continuing the PD films but feared it would not be possible due to Ms. Hathaway's new success outside of Disney.
Anne made it very clear back in 2004 (after she missed out on the chance to play Christine in The Phantom of the Opera in favor of her Disney contract) she was ready for dark and gritty characters.
A statement which she has proven to be quite serious about with her most recent roles such as Les Misarables and Intersteller and I find her desire to expand her talents to be fantastic. So I would not blame her at all if she'd declined to return to the now Queen Mia role.
I'm interested in finding out more about their intentions for this as they're calling it 'Threequel'.