(there is more real world difference in tech between the cold war era to modern than club to sword)
If you're comparing a random tribesman in skins to a trained classical swordsman with armor and a shield, then I massively, massively disagree. Its relatively difficult to kill someone in a single blow with a club - and to do so you have to bring the club waaaay back to get a good swing, which leaves you very vulnerable to being skewered by a sword.
Similarly, an 1870 rifle is massively superior to an 1810 rifle, and similarly 1870 artillery piece vs 1810 cannon vs 1510 cannon, but we don't bother to separate them in-game.
A 1975 tank can still kill a 1990 tank.
Hell, when do you think most of the 1990 tanks were produced? Its not like every couple of years they rebuild the entire US army from scratch. A lot of the tanks they have now a pretty old; they maintain them, and they add a few extra technological things, but the core model is still the same.
The M1 Abrams started production in 1980 and is still being produced today, 30 years later - slightly different models, but still basically the same thing. And its scheduled to be in use until 2050 - 70 years!
http://en.wikipedia.org/wiki/M1_Abrams
Also, don't judge the world by the US military, which far outclasses any other technologically. The Soviets kept pace in some areas (including tanks) for a while, and sometimes some of the European designs, but no-one else really did.
Human history is already represented in civ as exponential rather than linear with turns going from hundreds of years to just a few years.
Yes, but there's a limit to that. In Civ4 we basically had: ancient (chariot, archer, etc, roughly 1000 years), classical (spearman, swordsman, horse archer roughly 1000 years), medieval (knight, pikeman, maceman roughly 500 years), renaissance/enlightenment (musketman, curaisser 250 years), 19th century (rifleman, cannon, cavalry, 150 years), WW1 (infantry, artillery, machinegun, 30 years), ww2 (marine, tank, fighter, bomber, 30 years), modern (modern armor, jet fighter, mech inf, 30 years).
So we're already massively over-representing modern era, there's no need to shift the balance even more that way. Rather than differentiating between 1970s and 1990s, I'd prefer them to just add a ~2010-2050 future era.
The changes in warfare between 1970 and 2000 are really evolutionary, rather than revolutionary. Its just incrementally better versions of the same units. Precision bombing and now unmanned drones are the only really "new" advances in warfare that hadn't been seen before. And cruise missiles, depending on which eras you're comparing.