@anamur-- Actually, demand has not been consistent, it has gone up. Copper is used for so many things, from construction, to transportation and electronics. The demand for it is high, but is growing more and more because many nations in the world are still developing.
China, India and Brazil were under-developed nations several decades ago and look at how far they've gotten now. They're headed to be some of the wealthiest and most populous nations of the world. What this means is that we're going to need more and more copper which is going to drive prices up.
Even though copper might not be something that's going to disappear soon, many of the copper resources are yet to be discovered. So I don't think that we can supply copper as fast as demand is increasing for a very long time.