For me, though, this film could say "It's all a dream" and I would feel even more satisfied. Because the premise is "through a very complex dream, we can enact real change in a character." All of the sudden it's not a fake-out bullshit journey, if that's the case. In other words, if I'm satisfied by the success of Fischer's transformation, then Leo's growth is just as satisfying.
But he doesn't have to be dreaming for that growth. If, by way of example, in the last scene where Cobb ran off to hug his kids, there were a reflection of Mal in the window? That would make it far more vague and I'd say, sure. But that's not there.
Close your eyes and listen to the sound at the end. I really do think the top wobbles and that it's real. Cobb does go on a journey, because that's what movies are, and I think that's what leads audiences to this kind of speculation. Because of the story he chose to tell, Nolan is also commenting on the nature of stories themselves, all stories, which is why Leo's change can't be evidence that it's all a dream.
To me, the real story all boils down to Saito's line in the helicopter. Leo wants to go home and see his kids. Saito says, "I can help you, but it'll have to be an act of faith." Leo has to trust Saito, and he does this while putting total faith in himself and the team, and everything goes apeshit wrong, but he has to believe that if he does the job, Saito will do what he promised. And they've grown, they've become friends, which is why Leo says "Come back and let's be young men together." Leo's follow-through on that act of faith is his transformation. He becomes a person who can take a chance.
There's also kind of a beautiful negative symmetry between that leap of faith, and Mal begging him to make a similar leap of faith. After he did that with her, and the guilt plagues him, he can't function anymore. He's exploring his memories in a dangerous, unhealthy way, and he's going to let that go by the time the movie's over.
Everyone's so concerned about whether the top falls or not, but no one seems to care that Leo walked away without caring. The moment he sees their face, he can walk away. That's testimony to the fact that he's gained that faith.
