I always took it as more of a security issue. They made a point of mentioning that you shouldn't let anyone else know exactly how your totem works, never explain, don't let them handle it. Presumably because if they knew they could manipulate things if you were in their dream, to fool them into thinking their dream was the real world. It was the thing that actually tipped me off that the top wasn't Cobb's totem. They flat out said it was Mal's, they even explained how it worked. Meaning it was useless as a totem. I didn't pick up on the ring being his totem, but when that theory surfaced I thought it made sense. As for the ending, it made sense to me at the time and still does. The camera went to black before the top fell because the top wasn't important... and neither was the wedding ring or any other totem. The film ended there because it didn't matter at that point if it was the real world or a dream, Cobb was accepting that world was real... and nothing was changing his mind.