JP probably has the best perspective if Billy would be a great choice based on Billy's leadership, organization, recruiting, etc.
Personally, if Bill wanted Billy to be his successor it would have been best for Billy to become a head coach elsewhere to develop his own W/L history. ISU will have its pick of some very successful head coaches if the job does open up.
It kind of seems like everyone see's Billy as a TJ Otz type coach. Like yeah we really love you but go ahead and go prove it and we would glad to see you back coaching our team someday if you succeed.