"Already" is doing a lot of heavy lifting, by the time the game comes out it will have been 9 years since Civ 6. That's 3 years longer than the gap between 5 and 6 which is the next longest gap in the series. At this point they're vastly limited in the things they can do without a complete overhaul on base mechanics we've been playing with for nearly a decade.
Would you rather they completely overhaul the current game and the original mechanics be lost to time? Of course not, the obvious next step is to make a new game.