Thank you!
I understand all of the things you mentioned, but just like you're saying, it has to do with the strategy as well as the story. I think it's important that old resources are needed even in the later ages to force the player to stack up on them and manage them more wisely.
Not sure how I could let the player know what they need in the future - giving them a list of resource-amounts would make it a bit too easy :P.