I would venture to guess that some people who travel thrive on the connections and relations they make along the way. Chances are I've only met those types of travelers, because the opposite type was busy in the malls?
Like anything else, if you've done a LOT of one thing (say you've traveled 100+ countries, tasted everything, learned 2-3 extra languages), then maybe you get bored with travel for the sake of travel. Even still, that cannot be as mundane as spending the same period of time at one job in the corporate world.
I'm not a 100+ country taveler; I'm a 10+. And for me, meeting nice people and experiencing new foods is my lure. But once I get someplace, exploring the architecture, sampling the music and local culture, and meeting other travelers to share stories are the attractions. Likely 10 years from now I'll care a lot more about stuff that involves less travel.
So to your point, yes there is probably an asymptote. I bet it's far beyond what most people have experienced. And without being too peace-hippy-ish, I would argue that the more people get out and meet others from distant lands, the less conflict we would have globally. It's much harder to say the Chinese are evil Communist thieves when you've met a number of really great, kind, thoughtful Chinese people. (I just use this as an example.)
> The anticipation of travel was a more powerful driver of happiness than anything participants experienced overseas or conjured up in post-trip nostalgia.