Civ leaders aren't people; they're avatars. They're meant to make you feel like you're playing against someone rather than an abstracted mass. Changing leaders would go against the spirit of that far more than changing civs.
One of the very first things a human baby can focus on and recognize/react to is a Human Face. Even an abstracted human face like Wal-Mart's 'happy face' is more recognizable to a human baby than any other animal or thing. This is one of the very few things that seems to be 'built in' to all modern humans.
Which means we can identify and identify with Human figures better than any other image, icon, or construct in a game.
This is a huge advantage for the Civilization series, because the series has emphasized that human connection: animated Leaders, named Great People, Governors - far more than any other game.
I maintain: Humankind's mistake was not in switching Civs in-game, it was in the utter lack of identification with ANY of the Civs: no Leaders, no names, nothing but a generic Avatar made up by the game or the gamer with no connection to anything you were playing.
Which is precisely the difference between Humankind and Civ VII. In Civilization VII, that Leader, no matter how wonky he/she looks, is with you throughout. He may be Augustus of Slobbovia rather than Augustus of Rome, and he may look like a Schmuck, but he is your Schmuck and you will be surprised at how easy it is to remember that.
Or at least, that's the lesson from my Psychology 200 class at Penn State in 1968. Thank you, Doctor Foxx.
So everybody's concerns about It Didn't Work In Humankind, So It Cannot Work In Civ are so much Funglecarb: it isn't the same.
Furthermore, and contrary to many pronouncements in these Forums (including mine!) the Singularity Change between Ages and Civilizations is not Complete. You can choose or obtain Legacies that carry over, there are buildings that persist from Age to Age (possibly with different effects, I suspect), and you can even modify your Leader's attributes and bonuses (how, exactly, is still Unclear)./ Which means, simply, that you do not start with a Blank Slate in a new Age, and all of your efforts in the previous Age are not entirely wasted. You may be playing a Post-Roman Second Age Angleland, but there will still be some of your 'ancestral' Roman Civ buried in it. Exactly what and how you apply the Legacies will be very interesting . . .
In fact, I will postulate that the 40-page discussion after Launch will be on the best combination of Civs, Leaders and Legacies to give you the 'best' progress from one Age to the next. No, I'll go farther and do what I try to do very seldom: I will
Predict that Progression techniques between the Ages will be a major topic of post-launch discussion concerning Civilization VII.