True, But those are old tech. How many years would it take for the Burj Khalifa or the Eieffel Tower to ruined to such an extent?

Steel structures actually are much shorter lived. Without active maintenance, odds are neither tower would last a century. Generally the more advanced a technology is, the more fragile it is, and the more reliant on other advanced technology to maintain it. If you want something that lasts a long time, you're best bet is to stick to something simple like solid granite, though even if your artifact remains mostly intact, it's useless if it gets buried under sediment or weathered to the point that it's unrecognizable as an artificial construct.