This is the crux of the matter for me, why do we want to limit expansion??
Quite simply because every tactic, strategy, and mechanic in the game should have limits.
If None have limits, then the game becomes a boring Sandbox in which you can do whatever you like and likely will be bored stiff after 1.5 playthrus.
If only some have limits and some do not, then those without become the optimum play, decision-making is curtailed, and, again, the game becomes a boring Sandbox after a few plays.
And then is not played again, nor recommended to others.
The question is not should any and everything in the game have limits, but how are they designed, how are they implemented, and, as a footnote, do they bear any resemblance at all to actual events.
I submit that expansion, along with exploration, exploitation, and extermination, should be limited, and that the limitations should vary in severity and application throughout the game.
Good example: I just started reading a brand new book on the fall of civilizations (strictly by accident that it's so pertinent to Civ VII!) and have just finished reading about the Assyrian Empire - the largest geographical extent of empire in the world up to that time, which basically Always had problems keeping the 'expanding edge' of its empire from falling away. Even when a younger brother of the King was placed in charge of the sub-state of Babylon, he revolted, allied with an external enemy, and marched on Nineveh.
Basically then, from the beginning there have been serious limits on expansion - especially in Early Days, when the mechanisms for governing were pretty primitive and gaining legitimacy for a 'foreign' rule of any kind was nearly impossible.
Now, snap forward to Civ VII's equivalent of Exploration Age, and neither the Persian, nor Byzantine, nor any of the Arabic Caliphate governments had much trouble governing and maintaining control over a much wider area than the Assyrians had so much trouble with. In other words, the degree and severity of Expansion Limitation did and in-game Should change dramatically, at least between Ages and possibly within them (given the difference in span of control between, say, Assyria and Rome) as Technologies and Social/Civic Policies change and evolve.
And this should be matched by similar changes in limitations in every other mechanic in the game.