Technology makes resources obsolete by allowing new units and buildings that do the same thing without the resource. For example, rubber should not be required for Modern Armor because Synthetic Fibers is a prereq for Modern Armor. If you wanted to you could mod in pre-plastic and post-plastic units of all kinds.
In my mod/scenario "Ages" I have a tech "Horseback Riding" available in the Bronze Age that makes possible primitive cavalry units that require raw mustangs from nature or geldings from trades. When you get to the Iron Age you can research "Horsemanship," so all Iron Age cavalry does not require the Horses resource.
The rationalization is that the Horsemanship tech includes some animal husbandry and breeding pairs, so you can breed your own horses in captivity.
Also, I gave Horses a low number for Disappearance probability, so they tend to dissapear from one place and appear somewhere else, as real herds of wild horses will do.
Back when I played Age of Empires, before I boughtthe first Civ 3 right after it came out, I thought it would be great to have a Concrete tech that you could research, if you ever got to a modern version, so a new resource would appear that would replace Stone. Also, you could get new extraction technologies that make new oil appear, in Civ, but it would have to have another name, and you would have to make a whole array of the same old units, except using the new resource instead of the old one . Similarly, when I did my scenario, I included Copper, Tin and Iron resources. If you don't get either or copper or tin you can't make Bronze, and the chance of it is pretty good. But Iron is all over, and when you research it you can build a whole new panoply of equivalent or Iron units.
You can stay elegant.