I think overall I like strategic resources, but there are definitely some implementation issues that need to be resolved. It seems silly that I can reach a point technologically that I can launch a manned mars mission, yet be unable to defend myself because my people forgot how to make planes without aluminum or even how to ride a horse. It also seems silly that I can roll a huge Pangaea map with abundant resources and only have 1 oil resource on the entire landmass.
I like the idea of being able to build obsolete units (at least the previous in the tree), maybe at a production and upgrade cost adjusted for inflation of resources (to help negate the possibility of production of cheap units then upgrading exploit that would probably happen otherwise)
Maybe give this more expensive unit a slight buff for the increased upgrade cost? Maybe remove resource requirements for this obsolete unit? Idk, just tossing out ideas, try not to hate too hard, I know most of my ideas are bad