I am glad the pantheon still stands. But the real question is are there really any modern structure that will be worth preserving for 2000 years. Architects have long abandoned aiming for beauty to aim for originality. Originality doesn’t last. Humans in 2000 years will only wonder why on earth our society could manufacture atom level micro ships but couldn’t come with a single structure worth preserving.

Sand used to make concrete is a finite ressource, and extremely polluting to extract. Why wouldn't we save it and build lasting buildings?

I wonder that what we see preserved was only the minority of what was built at the time. Just as we do today, the ancients mostly built junk and then through the passage of time we only see their most durable work. I only wonder.

There are some structures worth preserving. Sagrada Família is one. Places of worship and private homes (at least those individually architect designed vs standard designs) tend to be a lot nicer than office blocks and other commercial places.

I do think a lot of new building hideous. Looking at St Paul's in London from the west so you see the hideous backdrop to its east is depressing.

Sydney Opera house comes to mind as something that might still be curious to people in 1000 years.

What makes you think buildings aren't going to change within the next 2000 years? Hell just look at all of the Tudor buildings dotted around. They probably didn't think they were anything special 600 years ago