You have to remember what its purpose is. As a kit lens for their pro camera bodies, the only real reason they would have to update it would be to make it cheaper to produce (increasing their profit on each kit).
And as others have mentioned, its better than any other standard walk-around other companies have, and while it has a few known issues, I'm not really sure they are easily avoidable in such a design (name me a 5x zoom Canon has that doesn't have HUGE barrel distortion on the wide end). Plus, would you really want to be paying $1500 for an update when the current version is easy to get at half that price?
And with them having just released both their full-frame bodies that would go with it, you've got probably 2+ years until another full-frame body where they would introduce a new kit lens.