ScarJo's casting isn't whitewashing, but an unfortunate reality in the film industry. The studio wants the movie to ultimately be successful, casting an unknown actress, in a property that's practically unheard of outside of the anime fandom, is just not an option for these people.
The Star Wars comparison was a good one, because that is a franchise that outshines any name. A franchise that can make practically anyone the next big thing. Daisy Ridley, unheard of until Force Awakens and boom, now she's getting calls left and right. She's the frontrunner for Lara Croft in the Tomb Raider reboot for example.
And you mention Christoph Waltz, yea, he broke out pretty big because he was in a TARANTINO movie. That's a good way to get noticed.
Ghost in the Shell is not going to make anyone a star unless they already were one.
Yeah I see what you are saying and in a lot of cases if a franchise/series is big enough they can make a relatively unknown actor/actress a star. However if the name/franchise of the movie alone isnt enough to sell tickets then in most cases it needs a big name to sell those tickets and attract that attention. Something like Star Wars sells on name alone these days and people will buy tickets regardless, simply because its Star Wars.
Something like Ghost in the shell simply isnt big enough worldwide to sell on name alone, so they need a recognised face/actor/actress to attract the attention to it. There will be rare cases where a relatively unknown franchise/movie will do well with a handful of 'unknown' cast members, usually if a movie/performances are really good & word of mouth spreads it but most studios wont take this risk if the movie budget is too high. Sometimes a director/producers name alone can sell movies (Tarantino was another good example). Studios want to make a profit and if there is any chance that they wont then they would rather spend a little more on a bigger star to get the attention to sell the tickets.
Rupert Sanders isnt a big enough name director to sell this movie on his name alone, and the franchise name isnt big enough worldwide to sell on that alone so they needed something else.