The German Panzer, Japanese Zero, American B-17+ Minuteman and the AA-gun wasnt historically long lasting units either. By the time we came to the 20th century the speed at which military technology became available, and became obsolete, vastly increased. So some of the newer units seems somewhat misplaces.
Like the jet fighter what does it represent? the jet fighters of late WWII or the today state of the art non-steath fighters?
Same goes for the modern armour unit.
But im confident that Firaxis will come up with some selution to it all... Otherwise we will just at them to have it fixed in a future patch