Oh you know, I did forget about the social policies from Civ 5 too. Well, yeah, you're right.
Maybe there could be a separate system of Artistic eras to reflect progress through the culture tree?
Realism Invictus actually does have a pretty good implementation of this. I understand that you advanced players of the base game aren't necessarily interested in mods for your regular play of 'IV, but I think it has value for a hypothetical game design discussion. It's not a perfect system (inasmuch as any mod is inherently confined by the original game's engine, after all) but I think it's a huge step in the right direction.
In the mod (and I'm not sure if you already know this, but in case you don't), there are 3 features of eras, instead of them being purely aesthetic as in vanilla: modifiers associated with the actual era change as it normally occurs, art eras, and definitions of statehood contingent upon the era in play.
When you enter a new era, both unhappiness and unhealthiness increase by one, reflected in the city screen with "We're not as happy/healthy as our forefathers!" I like how this loosely integrates an "instability from era shift" while having something of a realistic basis in truth. Even if not literally true in every case, the perceived nostalgia is definitely a real phenomenon, and there's a pretty good case to be made that continued urbanization and modernization haven't been inherently positive influences on public health. At the same time, each era offers even greater ways of increasing both health and happiness, so it's mostly about timing a slightly rough transition than punishing a sense of progress, which still occurs per all of the novelty of the new era.
There are also art eras reached within each proper game era, with a small infrastructure requirement. So, when you research drama and build a certain number of theaters, "classical art" becomes a buildable "idea/project" which boosts culture output in all cities by 10%. Likewise for the other iconic (usually Eurocentric, but oh well) artistic movements throughout history. The same tech which unlocks the building prerequisite for the art era project also unlocks several "great works of art" which are like wonders inasmuch as their availability is first come, first serve, and they are only unlockable by great artists. These replace the culture bomb of the base game, and instead output somewhat more culture than simply settling the artist (and also come with an additional token bonus for flavor and to distinguish them from one another), but the drawback is that they eventually expire, so that you have to make a strategic "now vs. later" choice, kind of like whether or not to wait for a math chop. It's a relatively simple work-in altogether, but it adds a lot of flavor (for instance, when you build the great works, you get a splash screen showing them IRL
) and a nice "feel" like, "Hey, my civilization just reached baroque art, and is now outputting just slightly more culture than those renaissance guys, all else being equal."
With revolutions and separatism (probably my favorite feature) being worked into RI, there are now notifications you get for a changed "definition of statehood," which is determined by the average number of civs having reached the associated era, rounded down. This definitely prevents era shifts from being purely aesthetic, since modifiers for separatism become typically become more and more punishing until the modern era (when "the end of history" occurs), but also modeled to reflect the driving historical forces of the associated time. So, for instance, the "age of faith" definition associated with the medieval era has the negative separatism influence from foreign religion greatly increased, while nationalism in the industrial reduces this aspect significantly, with foreign culture instead representing a huge domestic threat to stability. You use a combination of garrison strength, espionage, domestic culture, and certain civics to fight separatism, while these things' opposites as well as unhappiness and unhealthiness work against you. Really it's an incredibly depth-enhancing dynamic and I think Civ 7 should include something along these lines. There are some engine limitations with 4 which keep it from being seamless, but conceptually it's an excellent mechanic, IMO.