Obviously you have never spent much time around boats with internal halyards. Actually I have had internal halyards on half a dozen boats over 25 years and have never had the halyards slap inside the mast. I also have not noted chafe on the halyards within the mast. The part of the halyard within in the mast usually looks new while the part of the halyard on the outside of the mast most of the time UV degrades. I usually have replaced halyards that have begun to sun rot or which have been chewed up where they are cleated off. The only halyard that I ever had chafe on anywhere else was an external halyard that chafed on a spreader bracket when tensioned under sail.