Looking at this another way -- it's a new approach, an innovative business decision to release game modes for a game that can be used independently, or together. That's not an approach that they used for any of the previous members of the Civ franchise. Civ 5 had expansion packs and patches, but all were designed to work together. Similarly for Beyond Earth and Civ 4 and its expansion packs.
Was this new approach successful? They did sell a lot of copies of NFP. They might even be able to track increases in sales of the base game that correspond to release of the game modes; perhaps new people bought Civ 6, just for the ability to include zombies.
The new approach also introduced many opportunities for bugs to creep in, for game balance to be disturbed (especially in single player), and for other issues (bugs or game balance) in the base game and expansions to remain unaddressed. It's hard to say whether that is a blemish on the monetary success of NFP or not even relevant to the evaluation of NFP's success.
Perhaps the best way to measure success of Civ 6 is the ongoing play statistics from Steam. Are the numbers steady, indicating that a base of people still like the game well enough to keep playing? Are the numbers growing? Are the numbers declining, and how does that decline compare to other game titles in the months after their final content is released?
While I expect that Civ 7 will have expansion packs of some kind after the initial release, it will be interesting to see if they continue the trends of DLC that is integrated with the base game; game modes that can be turned on and off; and the balance between the two.