How would it dramatically destroy the movie? The term is existentialism. It may all be a dream, but that doesn't mean that you can't give your life (or the movie for that matter) its own meaning. I personally find this more liberating, than the idea that life is a game to played.
>It's already not real.
Dreams are real while we are in them.
Well... just off the top of my head, it would make the film insensible for a variety of reasons:
(1) It would make a mockery of what Nolan seems very clearly to intend as a positive ending. In the script he actually tells us he is centering the film on a simple, positive emotional message. So what is that message?
(2) It would create a glaring inconsistency with the symbolic landscape of the rest of the film. Case in point, the dream worlds are strongly associated with water symbolism, which even creeps into the real world when the dream world intrudes: it is a glass of water that sends Fischer to sleep on the plane, while Cobb's waking hallucination occurs while he is washing his face. And yet... unlike any other dream... there is no water at the end of the film. In fact, we have the exact opposite, since we are told the events take place in a garden on a cliff.
(3) An aside, but anytime you have people who are named after apostles frolicking in a garden with Dad, you should jump to asking yourself if there might be Christian imagery lurking there. So what's with all the biblical imagery, or the constant references to "leaps of faith"? Is it really accidental when characters blaspheme, or invoke religious imagery?
(4) the visuals of the children building castles on the beach would suddenly serve no purpose. There would also be no explanation for why Mal is supposed to be bad, when her name clearly suggests she is a malevolent character. Likewise, the names of James, Philippa and Ariadne would be meaningless. Ariadne's mythological role is helping Theseus out of a maze, so what is Cobb still doing stuck in one at the end?
(5) Cobb clearly develops as a person. Why does Nolan go to such pains to show this, and what does it matter if these changes accomplish nothing of significance? Which brings us back to point one, why doesn't Cobb just stay in limbo with his wife?
(6) This is a bit esoteric, but you'll get stuck arguing that Saito's palace is destroyed by water because Cobb was pushed into a bathtub rather than the opposite: that Nolan engineered the bathtub scene in order to find a way to destroy Saito's palace in a storm. This requires a violation of the principle of Occam's razor unless you're prepared to argue that there isn't really any water symbolism in the film, in which case you would be wrong. :)
>It's already not real. Dreams are real while we are in them.