Would you consider the style of ICS viable if maritime city states did not provide stacking per-city food benefits?
If your population is constrained by your happiness, absolutely.
1) The city center tile is extremely productive. It's a minimum of 2/2/1, but it will cost you an extra unhappiness point to 'work' the tile compared to the alternatives. Early in the game, this can be worthwhile, because of point #2.
2) There is no penalty for single-digit unhappiness when you are producing settlers. If you are REXing, you effectively have a free 9-point happiness buffer and can put off dealing with the problem until later, which are fixable because of point #3.
3) There are 3 happiness effects that scale to the # of cities: Meritocracy, Forbidden Palace, and Planned Economy. These are neatly spaced out in terms of when you get them, so if you expand exponentially at the right pace, you can hit these exactly when they are needed.
It's these three things in concert that make ICS work so well. Even without any Maritimes, a limited happiness budget will produce more gold, science*, and hammers with a large number of small cities than any other arrangement.
*A single size-10 city will produce more science than 10 size-1 cities. Ten turns later, 10 size-2 cities will produce more science than one size-11 city. REX will eat slightly into your research rate early on, but those losses are paid back fiftyfold.
Take away Maritimes, and ICS is still superior. However, Maritimes just make it absolutely silly.
Perhaps each Maritime ally should just boost every city's growth rate by (city-size)%