Fish are friends!

A shark would love this.

Survivorship bias. The ancient stuff that survived to the modern day are not more durable than contemporary engineering, they're just the 0.1% of structures that managed to survive this long.

The problem isn't that we can't build something that will last a millennium, it's that we rarely, if ever, need things to last that long. Nuclear waste storage facilities are the only thing that comes to mind. Everything else would need to be torn down and renovated or brought up to code at some point.

Hitler's flak towers are not going anywhere. There's other 20th century buildings which can last a thousand years with occasional maintenance, but those flak towers, nothing will take them down.

Most very old buildings that survived to this age, survived because the locals had a use for them and maintained them, or because they had a pyramidical shape. The colloseum was a castle, the parthenon a church, ... Without that usage, we'd only have the foundations and a few basements left.

I don't know. There's a bunch of giant statues that have been built. Buddhas, Guan Yu, Ghengis Khan, etc.

I have no idea if these were cheaply made, which I suppose is likely, but if they're concrete/stone, I could see them possibly lasting.