SPOILER****************************************
The thing is, if the ending isn't reality, then it really isn't as victorious as it should be. When Cobb parts with Mal, he makes a point to say that his projection of mal is not as good as the inspiration for the idea, an entirely separate woman whom Cobb's projection cannot fully capture. With that idea in mind, Cobb can justify moving past his projection of mal for the reality of his living children. If he does not succeed in fulfilling this idea, then the conclusion is pessimistic and Cobb's idea of the beauty of reality is contradicted.
Of course, a positive spin on this could be that mal is in fact performing the inception and showing Cobb the value of reality so that he can eventually free himself.