that may be historically accurate, but it will be an extremely Hard Sell to (most) gamers. [...]
BUT who wants to contend with cities and improvements and districts you have lovingly built up suddenly turning on you?
Civ1 had possible schisms for large empires. It IS FUN if happening to others, but unbearable to the human player happening to himself.
Heavy map modifications through game, newly generated powers to challenge you, in general scripts to keep game interesting.
a "need to solve the late game being unchallenging" approach. More civs in play earlier probably just means an even bigger domination steamroller
For years I have cultivated similar ideas now.
Within one game it is the best possible (or only) approximation, but the casual player doesn't understand that and intuitively refuses "being punished".
So we can have
1.) a short game design, which ends naturally
before exponential growth results in boredom. E.g. civ1 was quite lean: no hitpoints/health -- every combat had one unit killed and the other uninjured (& promoted, to veteran; no other promotions/experience). 3 yields: food, production and trade (distributed per sliders into gold, science, luxury) etc etc. Civilization Revolution followed this concept too (?)
We can have also
2.) a long game design, in which exponential growth runs unrestricted into wellknown boredom, the player can continue and grumble
or stop and begin a new game ... (special auto-stop versions of that are Jon Shafer's 'chopped-off-endgame' and smart designed OldWorld, which has such a curved space-time that we cannot see the event horizon of blasting powder yet

)
And we can have
3.) a long game design, in which exponential growth runs and is attenuated with well dosed negative feedback. It is a good, well balanced game. But it is not the game most (casual) gamers want.
Caveat: the game most (casual) gamers want is long, full of details, wonderful exponential growth, no hurting negative feedback and impossible.
Maybe a
chain of standalone games, which allow normalisation of powers in each stage, can help our Homo habilis on an
epic voyage fly to the stars, finally.
but it's still better than "never", and I personally don't know why or who's decision is behind the late release.
I neither, just speculate someone wanted to improve civ5's reputation, after the STEAM "straitjacket" and vanilla development problems.
Btw, I seem to remember in the very beginning of civ6 an approach from CFC members(?) towards Firaxis in order to coordinate help/info for the modding environment, which was rejected ... anybody remembers details or pointers?