Mount Rushmore isn't a structure but it will likely endure. None of US will be able to vouch for that, though.
You're probably right but I can see in a thousand years when the new Taliban of North America decide to excise the graven images from the mountain just as they did when they blew up the Bamiyan Buddhas in Afghanistan.
Kind of amazing with all that riff-raff climbing all over it.
Pesky photographers. It's like the want to take a picture of everything with them standing on it...:)