Any act of film/literary analysis obviously requires thinking about something after you've seen/read it. The harder you think the better, and what you need to look for when evaluating arguments are evidence of internal consistency, plausibility and authorial design.
So where is the problem? Nolan himself has said in media interviews that the significance of the top is that Cobb walks away. He's also remarked on the centrality of creation imagery in the film and explicitly stated that "there's a relationship between the sand castle the kids are building on the beach in the beginning of the film and the buildings literally being eaten away by the subconscious and falling into the sea." Funny how he even specified the beginning of the film in that quote. No-one is post-rationalizing those words into his mouth.
Frankly, there's tremendous power to any interpretation that leads one independently to the same conclusions the director later makes, all the while explaining away what are inconsistencies under other readings (why Mal is bad, why the continual biblical references, why there is so much damn water in the dreams, etc. etc. etc.). The fact that Nolan is using fairly conventional symbolism (water = death/subconscious) is just icing on the cake.