Has it really? I think they were introduced in Civ6 and there I was ok with them because you could still move freely around them. You were not limited by the official era in regards to what you could research or what you could build. If you were advanced enough you could already do things officially from later eras and if you did poorly you were behind and did not magically catch up at the start of the next era. Before that I think there were no eras and you could just move along as you pleased. For example if you went for religion but missed and early one in Civ4 you could beeline for philosophy to unlock Taoism if I remember correctly. If you missed that as well you could still go for Christianity or Islam and did not have to wait until the game allowed you to do so.
Yeah there is two sorts of era change : explicit and implicit. Even if previous Civs wouldn't have mentioned eras explicitly, you were still playing from caveman to space, with all the transitions between it ; you were discovering new technologies, unlocked new units, new buildings, etc.
And that's precisely the topic of this thread, as I thought I clearly mentioned in the OP : eras are now "mini games" by themselves, they are not continuous anymore. It breaks some feeling of
emergent storytelling. I'm not saying one couldn't do something more specific with eras, as they are there and can be a source of ideas, but I would here join up those who say it's badly executed. (honestly before writing this I didn't know what they meant by it, now I know : the idea of playing more specifically with eras is not bad in essence, but what they did to them is kind of counter-productive according to me, but in the same time I have no other idea than making them separated "mini-games" right now)
We can make a parallel with the denomination "4X" and implicit eras, so there could be 4 eras : you start exploring actively, then when there's nothing more to explore at least in your close vicinity you start to expand actively, then when you can't expand anymore you start to exploit actively (whatever it means, I think the exploiting part is permanent from turn 1 to 500 in fact), and then when you can't exploit anymore you start to exterminate actively.
Here, exploiting could be depleting resources and impoverish the land (chopping, polluting, transforming grassland to plains and plains to desert, etc.) and wars could become necessary in order to conquer more sane lands, occupied by hunter-gatherers, city-States, other civs that may have chosen a less destructive path but maybe being less advanced, etc. Note that all of this can happen simultaneously, because there is independent powers from the start, civs that may declare war on you, or your expansion being stuck prematurely (accelerated 4X), etc. It could be interesting to give independent powers more territory, as in nomads needing more space to live and not necessarily having cities just yet.