I watched this yesterday. The plot was innovative and at par with the concept of "the matrix". The producers of the matrix released an animation based on the matrix called "animatrix" which explained the concept in more detail than the movie could. I think nolan should release a similar thing cause the movie has left out many questions unanswered. Here is my interpretation:
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Mal and cobb are experimenting with multiple level of dreams. They get stuck in limbo. They already have "totems" to know they are in limbo. They grow old together for 50 years. Mal locks away her totem in a secret place in her mind. She chooses to believe that limbo is her reality. Cobb finds it unable to convince her to die to come back to reality. So he finds her secret place and breaks in and plants an idea that limbo is fake and that they need to die to come back to reality. They commit suicide under a train to come back to reality. But mal is still bewitched with the implanted idea. She again starts believing that she is in a dream and that she needs to die to come back to reality. She eventually commits suicide.
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In the last part of the movie, fisher is killed by mal. Cobb and ariadne drop down to another level to rescue fisher. During the kick, eames resuscitates fisher and "brings him back to life". They catch the kicks up back to the first level.
Cobb doesn't catch the kick. He drops in limbo and meets up with an old saito. They are several questions here:
1) Does saito know that he is in limbo?
2) Does saito have his own totem? The spinning top belongs to cobb.
3) Does saito honour the arrangement at that moment?
4) Do they kill each other to come back to reality?
I do not think you die from an old age in limbo. You just keep growing very old until you die an unnatural death which takes you back to reality. I assume they eventually died. We do not how long this limbo lasts but the sedative in the real world does last. Eventually, they have to wake up. When they do, the dream starts collapsing. And i think limbo too will start to collapse.
Fisher dies in the middle of the movie. Hence, i think the notion that the characters believed that if they died in a narcotized state, they would not wake up in reality is wrong. The characters would actually get stuck in limbo for unknown period of time. But when the sedative starts varying off, the limbo will collapse and they WILL wake up. What needs to be know is that the team went in to plant an idea inside fishers mind and they were successful. We do not know what happens with cobb and saito in limbo. Maybe they kill each other in agreement at that moment or maybe they grow more old together. But when the sedative varies off in the plane and when they wake up, both are confused, but get signs from the other team member that the idea was planted. Saito honours his agreement.
The screen turning black just before the metal top stops spinning was unnecessary in my opinion. It only confuses those who had lost the plot in the middle of the movie. My friend who works for fame adlabs says a noticeable number of people went home after the interval itself.