Many times the fact that people feel the need to "visit places before they're gone" is the very reason they'll be gone or at least diminished to such an extent that they are vestiges of what made them great to begin with. This is particularly true of such naturally wonderful sites as you mention in your post above.
Mt. Everest is a particularly glaring example of a wilderness reduced to a freeway of tourists with their ever-increasing demands on the environment forming lines to have their shot at the mountain. I haven't been there myself but a close friend bought a tour to Everest Basecamp and was appalled by the trash and crowds along the way and was forced to contribute to the decline himself, to his horror, when bad food made frequent trips off the path necessary, there being no alternative, in full view of the passing hoards. Turning his back to passersby was the only semblance of privacy available.
One cannot stop the ravages of time but limiting the numbers of visitors, as some more enlightened governments have started to do, may be the best way to prolong the lives of vulnerable places. I think Bhutan has the right idea.