It's the zoo problem all over again.
Animal-rights advocates frequently argue, not without some justification, that it's unethical and exploitative to capture wild animals and lock them up in cages so that we can gawk at them. So let's follow their advice in a hypothetical sense and get rid of zoos, circuses, even pet stores. Fast-forward a generation or two and let's imagine how that might turn out. The people from which those very organizations draw their membership will have never seen an elephant or a tiger -- or, for the city-dwellers among them, even a deer or a rabbit -- in real life. These are also the same people who vote for Congressional representatives to pass or repeal conservation laws.
In this hypothetical future, volunteers from the animal-rights organization will go door-to-door canvassing for votes or petition signatures, just as they do today... except no one will have the faintest idea what they're talking about. The activists will find that they might as well be asking their contributors to save the T-Rexes in Jurassic Park. What's the difference? They're all just pixels on a screen.
Tourism works the same way, IMO. It may have harmful aspects, but the effects of completely eliminating it will be worse.
And decrying "mass tourism," as Wallace puts it, is nothing but code for "Only a few wealthy people should get to experience this."