Now, granted, the "good" about a game like civ is that since it's out there, and it doesn't rely on some online server infrastructure or anything, they could "shut down" the game and people could still play it. And the marginal cost to put out DLC content probably means that even if the game is being trimmed down, they could probably keep enough people on to put together enough to release some DLC packs.
The problem though I would think is that if the game just doesn't perform up to par on the long term, they lose enough players that the DLC aren't profitable, etc... it's more likely that the studio just wouldn't have the runway to really run a long development cycle for like a civ 8, and would conceivably shut down. Sid himself obviously isn't doing the hands-on work as much, but it's not inconceivable that he goes off on a full retirement with this iteration of the game, and if the studio isn't confident that the next iteration would turn things around, they might rather just not have another iteration.
Now, granted, I don't actually know the internal structure of the studio. So there's certainly always a possibility that 2k looks at the books and doesn't want to pursue a Civ 8, but the Firaxis team still do, and they either reach a deal, or sell them off, or the employees all leave and start a new studio, or whatever. But in this day and age, with how expensive some stuff is, one bad year or one bad release could be the end of the line for anything.