I've been thinking about ICS. Take a regular civ. We would want each city to have the Industrial, Commercial, Entertainment, and one other zone appropriate to our victory goal. We'll need every city at 10 pop for this, and a fully upgraded Entertainment Complex provides for 11 pop without considering other Zoos and Stadiums in range. Housing will come from City Center (1), Granary (2), Aqueduct (6 or 2), Factory (1), 4 Farms (2), other sources (policies and religion etc). At 10 pop, we will need 20 food, and working from plains tiles forming a 4-tile formation, we go from 2*4=8 food to 3*4=12 food to 4*4=16 food as tech progresses, plus 2 from the City Center, 1 from Granary, 1 from Water Mill, and we would get 20 food. In total we will need 4 tiles for districts, 4 tiles for farms, 1 tile for the Aqueduct, totaling 9 tiles. So that's the first ring plus 3 tiles from the second ring. Considering that we will more likely than not be forced to have more tiles due to water sources and terrain formation, we'll most likely be able to throw in another district (and have the food for the population) as well, to take care of our Encampment and Harbor needs.
It does seem doable, and we can always just find spot for our super production city with a good +5 or +6 adjacency Industrial Zone and a river for the Ruhr Valley.
If each baby city is in range of 6 Industrial Zones, it'll be getting 42+2+1=45 and any other production from tiles. So that's 15 turns per Mechanized Infantry, but we'll be rolling out lots in parallel. If we have 15 cities all doing this, it'll be 5 Mechanized Infantry Armies per 15 turns, or on average it takes 3 turns to train a Mech. Inf. Army. Even if we consider other victory conditions, simply having that many cities and that many districts geared towards our victory goal, makes ICS a lot better than any "tall" empire. 15-20 Theater Squares, 15-20 Holy Sites.
With Amenities and Housing basically taken care of by each individual city, it seems clear that the problem is with the ranged production buildings.