I think 1:1 steam engines will be running for as long as the metallurgy allows them to, which I suspect is at least the next hundred years. After that, there will start to be major failures in things like the frame, which are not easy or cost-effective to repair. I think the chances of a running steam engine five hundred years from now are very small. The ringer in all this is the "green" movement. Even if we have steam engines a hundred years from now that are still capable of running, will we still have oil and coal to fuel them? Will they be allowed to run under more stingent (some might say draconian) pollution control laws? Time will tell.
