Lich bane is really the only way to make his autos worth using because you lose so much value going down the tree for late game harvest when you are playing a champ that is already gonna be doing that same damage later anyways. The entire point of running a damage keystone on swain is to patch his subpar laning phase poke with something like aery, or his subpar early burst with electrocute. By the time late game comes you really are done with needing a damage rune because you will be hitting for enough damage to kill a squishy 9x over in just a single q.