You're right that it can work for other games. I enjoy Old World and CK3, but they're both very different from Civ (even Old World, which is closer). The personality of the leaders is a big part of Civ's appeal to me as a franchise. I love to hate Dom Pedro in Civ6; Jadwiga feels like a little sister; etc. It's not just about the relationship in the game but across games. I think leader switching would diminish that. It's worth noting that the games where it has worked are essentially dynasty management games (CK3), dynasty management minigames (Old World), or games where the leaders simply don't matter outside of a few numbers (Stellaris).