No, it's not illegal to own an old VHS. Lucas has implied in interviews that he won't be sorry when the last tape wears out. He wants people to forget the originals. Even Laserdiscs may be unplayable someday when it's no longer possible to get a player fixed.
Even the 1997 versions are fading from people's memory. As is the theatrical cut of Episode One. Neither of these has ever been on DVD as they were in theaters.