First off, none of those guys are bums. Second, you are right that this year none of them are better than a healthy Conley... but it is not about this year. Over the last five seasons he has averaged 55 games a year and that probably isn't going to improve with age.
Would Conley make the Jazz better this year? Yes, but maybe not the next three (which is when we should be focusing on). Also, this move is not such a no brainier that we should make the move before knowing how everything is going to shake loose with AD.