Actually this and what you're talking about go hand in hand so you sorta have to talk about it because what you're saying about them being behind barriers and guarded is because of that attitude. You can't get anywhere near them because of that.
Now, obviously no one wants these great natural wonders to fall into ruin and not be appreciated so some form of tourism is needed, especially for things that need a lot of upkeep. this is why over here a lot of old grand manor houses and castles are now open to the public, because it costs a fortune just to keep them from collapsing, never mind in some kind of historically appreciative state. I'd assume a lot of other wonders do need some kind of up keep that isn't cheap. So there is an element of commercialism needed.
Now, people have been defacing and vandalizing things since the beginning of time, that is not a modern thing. We have ancient Roman graffiti preserves in places like Pompeii. So that attitude is not new. Relics, aka stealing things from dead people or sometimes even the dead people, has been a religious thing since forever too, after all. In many ways pilgrimage was the original tourism and I'm sure a lot of people were doing the olden days equivalent of what we do now, but since everything was slower, it was slower. There's a relativity that has to be taken into account.
There is of course, the argument that ease of access and cheapness cheapens the experience. If you want them to be revered, you have to make them special. Make them cost a fortune, make it so they're not easy to get to.
But also, I'll admit some things just aren't worth the hype. Things like the Mona Lisa (I know I'm a heretic) just aren't that interest, and I'm not sure why it's considered so beautiful, so I was sorta meh when everyone got in a fluff over whoever it was just went there to take selfies. How does that tiny little painting that isn't that much more beautiful than a lot of other paintings of similar time ever live up to all the hype? And there are plenty of things like that. I used to live near Stone Henge, back when you could get near it before it was taped off because of graffiti. It was meh. Venice, unless you like canals a lot, is meh. We went to see "voted the most beautiful city in the world" in the Canary Islands, drove up hairpin bends on cliffs and you know what? It was a few houses and a few shrubs over looking the sea! It was awful. Compared to the scenery we drove through to get there, including the stunning barren lands of Mt Teide.
The problem is, the modern world is changing and if these places can't keep up they will die out. I used to live in a seaside tourist town. We made all our income in 3 months of the year. Do you know what would happen to that town if we couldn't keep up with what the tourists wanted? The same thing that's happening all over with seaside towns; people have to move away, the spare houses are filled with people who need government housing but none in the cities so they're sent to fill out these spaces, but there are no jobs in the area so the entire town becomes a town of people without jobs. Now, perhaps it would take longer and not be as extreme in some these big places, but if you don't tell people to come, they won't come and tourism is a driving force in many economies.
Perhaps that's not the right or moral thing, but I don't think we should hate people for trying to make a living and the people who provide that living. And it's the way it's always been, lets not forget a lot of ancient landmarks were created so people would come and see them and spend money.
There are plenty of nature wonders left over here, idk about where you are. We have entire huge areas dedicated to the natural beauty of things and most of the tourism industry there is B&Bs so people can appreciate the unspoiled natural beauty of it. There are no guys in Hawaiin shirts, there are people in walking boots and coats preparing to hike the hills or go boating on the lakes. They're people there to appreciate the beauty and I'm sorry if wherever you are isn't like that.