The ending reminded me very much of the one in "Taxi Driver" in the sense that the movie finished on an emotional level. At the end of Taxi Driver, we get a collage of newspaper reels stating how Travis Bickle is a hero for taking the law into his own hands and rescuing a child prostitute from her pimp. The world loves him but yet we don't know if he's alive or not from the shootout or if it's his thoughts before dying but at least we assume that he's seen as an honorable man now.

Based off of that, what difference would it make if Cobb was still in a dream state or not? The plot of the film (the intellectual part) revolved around Saito's Inception scheme but the story (the emotional part) revolved around Cobb's torment and like Taxi Driver, it ended on an emotional note than a literal one.