There are not many options to go beyond FE. The game already dragged out near future in two eras TE and FE, i can not see many options. Colonization of arctic regions was one of the futuristic fiction scenarios, so now they went to colonize underwater areas. If you think about it, colonizing just 200 meters below sea level is much more sane, than launching shuttle to Mars. Inno clearly will never go in space direction, since at that point all logic would go haywire, so back to earth underwater construction and desert colonization are logical. I can see the argument that giving the city half submerged looks can be called post-apocalyptic, since you had the city and now it is under water. If you look at reality, at least 1/2 of your city will be on normal dry land and will not look like the one on video, thus giving the idea of expanding the living space for a coastal city in water direction.
As to underwater wars. It is more reasonable to make a dome with air to dig for resources than dig with submarines, some will try to take advantage of that so army guarding harvests is needed. The vehicles are not mechanized animals but implementation of naturally evolved ergonomic shapes of animals to underwater weapon design. If the bubble bursts such units can still go on, normal tanks can not.