In your son's case, since he obviously didn't have anything remotely relevant or collectible to have him sign, the selfie was probably the best move. (Unless you happen to be building a collection of signed airline barf bags! Lol) Also, given your son's age and his generation's love affair with the cellcam and the insta, the selfie was probably more meaningful to him anyways.
Since I'm an autograph collector at heart, this scenario is the very reason I never board an airplane without at least having a blank baseball, a few 3x5 index cards, a ballpoint pen and a sharpie in my carryon bag.
PS. I also agree with your original thought...whenever I'm at a banquet-style dinner and the server says, "steak or chicken?" My standard answer is "both please!"