You ALL missed the point - MAJOR SPOILER
I've read all of the notes on this board, and I'm afraid to say that every single person here missed the point of the movie. This is not attacking anybody's intelligence, it's just that I haven't read a single post that has come close to discovering the true subtlety at the end of the movie.
I notice in the comments section of IMDB that many people expressed their dissappointment about the ambigious ending... was he, or was he not Don Juan? Many people said the movie sucked because they didn't want to decide for themselves. Ok, valid opinion. However this proves that the film-maker has gone right over their heads - for he HAS provided a definite ending. They just didn't recognise it. I'll ellaborate...
Many people here have theorised that Depp is, and many people that he isn't, the real Don Juan. Actually, this debate ALSO has nothing to do with the point of the movie.
Our first clue is given in the "grey husk" speech. Don Juan says "It is only in my world that you can breathe", to which the reply is "You are right. My world is not perfect". This clearly demonstrates that Jack finally ADMITS that the magic has left his life. It also demonstrates that Jack needs the dreamworld of Don Juan; he is captivated by the love and he realises that it is irrelevant whether Don Juan's reality is factual or not... he realises that the world is actually a better place with Don Juan off medication.
Ok, now skip forward to the "Why not?" comment. Many people like it because it provides an ambigious ending. It illustrates the point that it is irrelevant whether Don Juan is the REAL Don Juan... it is enough that the possibilities exists.
This is good, but it's only half of the answer.
People have been captivated by Don Juan's dream world and some say it's real, and other's just fantasy. But to date, all the of the fantasy elements in the movie have been supplied by Don Juan. He has been the dreamer and Jack has followed along. But think about what is going on when Jack says "Why not?". For the first time, JACK is determining the course of DON JUAN'S fantasy, is he not? It is NOT Don Juan that determines if he meets his lover; it is JACK who determines for us whether the lover is waiting on the beach or not. This indicates a subtle shift that we have left Don Juan's fantasy and entered Jack's. The "Why not?" comment at the end is NOT designed to resolve Don Juan's fantasy. No, it is there to demonstrate that the viewer is now in JACK's fantasty. HE alone now has complete control over what we, the audience, believe.
So you see, the "Why not?" comment DOES provide a definite ending, but not in the way that you first realise. It is NOT an abiguous ending. The "Why not?" comment can never be replaced because it completely makes the movie. It demonstrates the full transition of the viewer from Don Juan's fantasy to Jack's "dreaming" power.
It is COMPLETELY irrelevant whether Don Juan's fantasy is true or not... it is now relevant that we the audience like Don Juan's world so much that when he leaves our focus (as the camera pulls back), we are now WILLING to let somebody ELSE dictate how the story goes.
The point of the movie is NOT to get you thinking about whether Don Juan's world is real or not. The point of the movie is that ANYBODY has the power to create such a world. Even an ordinary schmuck like Jack... and hence by association even us, the audience.
This shift is so subtle, and it is cleverly disguised behind the OTHER meaning of the words "Why not?". I take my hat off to Jeremy Levon.