Agreed about sales definitely will cause extra civs to be created. It would make economic sense to them to do that. Question is where the spot goes where they can make the most profit. Too expensive and there may be very few who buy. Too cheap and even though many sales the revenue can be not so good. But there's an ideal middle point somewhere. And it is probably lower than now. But who knows.
The description of how long it takes to make a single new Civ & leader is really lacking. Consider the following.
- Decide which civilization.
- Decide which leader.
- Decide traits that suit the civilization.
- Decide traits that suit the leader.
- Composer(s) to do the peace & war scores & any unique music for scenario.
- Research & write dialogue for leader.
- Do casting for voice actor for leader (must speak a similar enough language fluently and so on).
- Schedule recording session in studio with voice actor and sound engineer(s) and actually do it.
- Research the leader and civilization and have writer(s) do the Civilopedia entries.
- Art director to decide general direction for the leader scene and the two unique units/buildings.
- 3D artists build the meshes for leader scene and the unique units/buildings.
- Texture artists do the textures for the leader scene and two unique units. Shaders especially may need extra work.
- Illustrator(s) do the 2D interface artwork for Civilopedia and unique unit/buildings.
- Animators animate the leader, horse (in this case), unique units and buildings. Just think of the speech, facial expressions, gestures, unit idle animations, fighting animations, movement.
- Sound engineer creates the sound for the leader scene and unique units and makes it play at the correct instances (syncronized properly).
- Programmers add code/scripting for all of these new things (a lot of stuff!).
- AI programmer/Game designer(s) create the AI personality suitable for the leader.
- Map maker(s) make the world for the scenario.
- Game designer(s) set up & tweak the scenario.
- Testing, balance concerns. Q/A. Smooth integration delivery through Steam must be set up.
Even listing these things is exhausting so I'm not going to thinking of what else is missing from it. You can imagine that these things cost real money - paying all those professionals to do all that work. Game creation is a very complicated expensive task these days.