She wasn't the lead character though. I think it's easier to get away with recasting supporting roles.
Despite that though, I agree that the general public are not as stupid as some people would have you believe. All this worry and hype about whether keeping Routh in the role would be confusing or not - you know what? Once asses are in cinema seats and Superman is on screen doing his thing, no-one will give a damn. Once a movie starts you tend to immerse yourself in it and as long as the marketing and PR machine make clear that this is a new take on Superman, it won't be a problem.
It's a very unique situation, I'll give it that. You have this capable actor who's the right age for the role, has the right look and build for the role, wants the role, and has played the role before. An actor who's face was on every bit of merchandise associated with the role and who many people still see as 'Superman' - that's how he's introduced in just about every interview, magazine article and TV show he's ever on.
And whilst the film he was in wasn't a critical success, it still made a lot of money - don't confuse the relatively low profit made by the film as a sign of failure. Plenty of people saw the film but the huge budget ate into most of the profits.
Yet because the film's director chose to take the entirely wrong direction with the film, we have this situation where the studio want a new movie which is totally distanced from the old one.
In a way, I liken it to the James Bond movies. After the release of Die Another Day with Pierce Brosnan, the film was criticised for dodgy effects, questionable plotlines and silly gimmicks. An invisible Aston Martin??? It became clear that a new approach to Bond was needed and Casino Royale took a much more realistic, edgy and gritty approach to the franchise, with great success. But with Daniel Craig replacing Brosnan in the role.
The question is, putting aside age and physicality ........... would the public have accepted Brosnan returning to the role, playing the same character in a much different film?
As long as he played the character differently - in a serious manner more suited to the style of Casino Royale - I think they would have. This is the closest comparison I can think to the current situation with the question mark over Routh's recasting.