Life Expectancy of Steam Locomotive?

How long can a steam locomotive last with proper care and maintenance? What parts would be the first to go other than normal maintenance items? What parts if they went beyond wear limits or broke would permanently put a locomotive out of commission? How much do the cylinders wear and can you put in new pistons and rings as in an internal combustion engine? I am a mechanical engineer in the auto industry and am just trying to get a frame of reference compared to automobiles. As far as castings and the like goes, since we cast millions of cylinder blocks and cylinder heads every year I assume that it is probably a matter of money (up to a certain size limit) as to replacing large cast parts goes.
I'm not advocating that we stop operating any historic locomotives. As a matter of fact I think that for people to truly appreciate steam locomotives they need to see them operating. I'm fairly young (30) and never cared much for steam until I moved to Denver and started volunteering at the CRRM. Now I'm a true steam fan and NG fan after having been involved. I hope more young people can learn to love it and I don't think they ever will just seeing one sit in a museum dead.