Why the ending IS real-- and it has NOTHING to do with the top
Remember the scene in the outdoor cafe in which Cobb tells Ariadne that in dreams it always seems that we are "dropped" into the action and that we can never remember the "beginning" or "how we got there"?
Admittedly, the last scene is very dream-like-- the warm lighting, the children wearing the exact same clothes and playing in the exact same position-- but does Cobb just "show up"?
Not at all. In fact, the preceeding couple of minutes to this last scene are visually the most banal and NON-dream like: Cobb wakes up on the plane. Cobb waits in line. Cobb goes through immigration. There's that extended shot of Cobb and his team and Fischer waiting for luggage at baggage claim (how exciting). Then Cobb sees his father in law, who is there to pick him up and take him home.
What's the point of Nolan showing all these banal, everyday aspects of airport routine? To show there is a logical, normal, decidedly non-dreamlike process-- beginning-- to how Cobb ended up at home, seeing his children. The ending is real, and you could argue it has nothing to do with whatever ends up happening to the top. There's no faceless corporate boogeymen, no walls closing in, no Mal popping around, no staircases that end in other staircases: it's a plane at touchdown, an immigrations check, a baggage claim shot, a guy waiting to pick up his son in law at the airport. So the audience knows-- and Cobb knows-- "how he got" to his children at the end. That wouldn't happen in a dream.
And there'd be nothing to argue if people weren't arguing back. Zefah in particular was and perhaps still is adamant that it isn't Limbo. 
