The point is, while it may have started out as a ligature (of either ſs or ſz, no one really knows for sure), it has long become a letter in its own right. You

The point is, while it may have started out as a ligature (of either ſs or ſz, no one really knows for sure), it has long become a letter in its own right. You cannot treat it like a display-only ligature without throwing away information, e.g. the difference between Maße (measurements) and Masse (mass). People in Switzerland made a conscious decision not to use ß anymore, but that's not the case in other countries where the language is used.