What's realistic and historical is
both "tall" and "wide" together: a tall core (think imperial capitals like Rome, London, Vienna, etc) supported by a "wide" periphery of less developed territories.
Real historical (and current) empires have worked by concentrating wealth from the periphery to the capital and heartland. Think of it as a pyramid shaped empire, as opposed to the flat-topped trapezoid of most Civ games (Civ 6 included) where nearly every city is on equal terms and able to become almost as highly developed as the capital. Or the silly "tower" shape of the Civ5 four city tradition game. The capital is the tall top of the pyramid, while the wide base represents colonies bringing in luxuries, breadbasket regions bringing in food, military outposts, etc etc. Smaller, more specialized cities and regions.
Realism aside, does it matter for gameplay? It's subjective, but to me it does. I find the empire of almost identical and equal cities (late game) to be quite boring. The players asking for more tall play are yearning for some system that awards them for building a super capital.
For this kind of "pyramid" gameplay to work, the game needs the following features that Civ 6 currently doesn't really have:
- A major reward for building up a core of one or more big cities (better science, culture, production, whatever).
- The way to build up a core requires resources (food, luxuries, etc) to be transferred from other cities, limiting their development.
- The main benefit of territorial expansion would be in securing resources to grow the core.
- Note that the pyramid can still have a middle, meaning that large empires might have still some regional power centers separate from the imperial core (e.g. conquered capitals).
We can still enjoy Civ just fine as it is (and always has been) but I'd love for the game to move more in the direction of this more realistic "tall and wide" model in the future.