I'm guessing this is how they are going to say: Look, here is 8 more classes, and we know you are now playing 20th level characters and don't want to restart or abandon your character, and you've already picked a multiclass, so here is how you use these new classes with dual classing.
I'm not so sure. If its one thing I've seen designers do in this edition, its that they are looking at older editions of D&D and disecting them and evaluating whether they can add components of those editions to this edition.
I for one, am looking forward to dual classing.