Actually haiku has already implemented most of this … It does not yet have rounded windows but I believe this is a planned feature it may get pushed to R2 however
When you are getting things done the UI appearance matters little. However the Haiku UI has been recently updated with gradients and a generally smooth uncomplicated appearance that many people like. As features are added to Haiku the UI will change and improve to reflect those changes as long as they allow the system to remain lean and fast. I think that is the general sentiment of the developers and long time users besides you kind of get attached to a cool different UI like BeOS/Haiku’s after awhile.