Or, alternatively, Firaxis recognizes that changes made to the game post-Civ 3 have either (a) fractured the community or (b) expanded the total audience of potential Civ players, depending on your viewpoint. Some people prefer Civ 4 and would pay for more development and content for that engine. Some prefer Civ 5 and would pay for more development and content of that engine. Some prefer Civ 6 and would pay for more development of that content.
If Firaxis decides to support only the Civ 6 engine going forward, they'll reap greater sales from that market, but not from the people who are looking for a different Civ experience. Civ 7, on the other hand, gives them an opportunity to both expand the potential audience even more, and up-sell to people who like earlier versions and are left cold by Civ 6.
Finally, I don't buy the idea that Civ 7 needs to be faulty on release. Civ 6 wasn't really faulty on release, it was just … Civ 6. If you liked it, great. If you didn't, you spent a lot of time (like I did) trying to figure out what was "wrong" with it. Only to eventually realize there was nothing wrong with it. It was (and is) the game the development team wanted to make. Not a game I enjoy playing, but that's on me for wanting a different gaming experience than Civ 6 offers.