I think this is one of the systems where we shouldn't really go for a lot of realism. Some more of less serious posts in this thread show how absurd it could quickly become (cotton or silk to be able to make clothes - and thus ANY unit; to make steel you need not only iron but also coal etc.).
I'm completely fine with the principle that each resource is mostly tied to few eras - to those eras when that resource was used to make the most hi-tech stuff (when horse-riding was the most advanced thing you could do, when iron sword was the best weapon, when gunpowder allowed rifles etc.). Sure, iron has been a crucial part of almost anything since the beggining to these days - but for gameplay reasons I don't think it should be required in the later eras. What if you were not lucky and found no iron? You had a hard time in the classical eras, but why should you be punished till the end of the game?
Adding some different use to the obsolete strategic resources could be a good idea (like turning them into some kind of amenity etc.), but IMHO too complicated. I'm happy with the current state.