One question I wonder that I don't quite know how to answer- perhaps the important part is how does tourism compare to the other hobbies people might engage in to fill their time. It's probably worse than intramural sports, but how does it compare to automobile racing or what have you. Or, how does modern tourism compare to tourism of ages past- is ten thousand people taking a photograph better or worse than a couple British guys shooting a lion?

You could also speculate about much harder to prove thesis. For example, if travel & "global citizenship" helps avoid a great war, that's quite a lot of damage & destruction averted.