I was thinking about other people who could have been Daisy Duke and one name I am thinking of is Dana Delany, the star of "China Beach". She was 22 when "The Dukes of Hazzard" was cast, so she could have been cast. However, there are things that could have prevented her from being cast, besides Catherine Bach. Ms. Delany was barely out of college at that time and didn't have any experience in front of the camera as far as I know. It didn't need to be a handicap since John Schnider didn't have any, as far as I know, behind-the-camera acting before being cast as Bo Duke. But I believe that Ms. Delany wasn't mature, in acting at least, to carry a leading role in a television series at that point. I believe she needed some more seasoning as an actress before she could carry a lead role, like she did on "China Beach".
On another note, here is a picture of Dana Delany in jean shorts in a deleted scene of China Beach's season four episode "The Big Bang". The scene is on the season four DVD.