The internet didn't exist and travel, perhaps especially exotic/adventure travel, was a much more rarified thing--so National Geographic was this window into a world that the vast majority of people would never encounter anywhere else.
exotic/adventure travel is even more rare nowadays. The rise of social media has funneled most travelers towards a few high-infrastructure hotspots like Bali, Yellowstone, and Barcelona. Historically, travel was performed much more 'blind', rather than following the internet-registered locations and reviews.
I don't know the effect in absolute terms but certainly there's less terra incognito from the perspective of Westerners than there was when the Wheelers founded Lonely Planet.
