The developers have listened to the community.
In Sid Meier’s Civilization 7, matches are divided into distinct “Ages.” When you move between them, certain settings (for example, treaties) are reset and players are given the option to select a different civilization.
The team originally intended this to motivate players to see a match through to the end. However, feedback and online metrics (with Civilization 6 outperforming the new entry) indicated that series fans prefer to play a single civilization from the opening turns all the way to the game’s conclusion.
Firaxis Games has now confirmed they are working on that capability:
We’re running internal tests of systems that let you play the same civilization across all Ages — allowing you to choose a civilization from any Age and guide it throughout your entire journey through history.
Source: iXBT.games