There were hundred of ancient cities placed in the coast due to the food provided by nearby fish banks, greater possibilities for trade due to harbors, and better defensive position due to cliffs (see also: Ancient mediterranean cultures such as the phoenicians or the Minoics) with the mouths of rivers being the quinquaessential prime location for cities and were the first civilizations flourished (the Nile's delta, banks of the Euphrates, Yangtze's mouth, etc).
Also, do notice how the vast euroasian steppes of Mongolia, Turkmenistan and the likes are not exactly burstling with human megalopolises, not even nowadays. Extensive grassland plains are not a good place for locating a city in the real life.
Yup, on board with everything you said there. The problem is I did say the major coastal metropolises people use as examples of why coastal cities should be better which all primarily saw growth from being trade hubs. I have never said coast is bad in real life. Only that the big coastal cities grew because they are trade hubs.
Also straight plains tiles with no rivers or bonus resources that would represent the Mongolian Steppes don't generally create the best cities right away either. You usually need some wheat, grassland, or a river to grow those very big. In game they do get pretty decent later on. They won't grow any better than coast with fish at any rate.
The thing is, regardless of the complaints, coastal settling is very viable in the game. It will probably be very rare for any city to use every available tile inland or not. A city founded on straight coast will lose ~16 tiles of the 36 it can normally work unless you're founding on some snakey peninsula. Most of the time a coastal city will be settled near 2 or more sea resources (at least that was my rule of thumb in V). So 14 useless tiles, reduced to 13 by the harbor district. On average a coastal city will still have 23 useful tiles.
If you hit a pop of 23 (to use every tile) you'll have 7 districts and 16 tile improvements. Fish and the harbor both appear to be pretty good sources of food capable of supporting whatever Civ VI's version of specialists is. Its pretty reasonable to assume a coastal harbor city will be able to support 7 specialists.
They don't have the same top potential as an inland city but 23pop wasn't a shabby sized satellite city in CiV. With soft pop caps being added back in with housing and appeal I'd be surprised if that's a small city in VI. Speaking of appeal, inland cities have incentive through adjacency bonuses and the national park mechanics to not develop some land. Since coastal tiles add appeal you can assume that coast replaces the need to avoid development.
There are many, many possibilities, me thinks:
Fishery: Unlocks at sailing. +1 food to every sea tile
Drydocks: Unlocks at shipbuilding. X2 to the yields of workboats and atolls
Sea batteries: Unlocks at gunpowder. Your city gets a second extra ranged attack that can only target sea units.
Waterfront: Unlocks with urbanization. Increases appeal in all your beach and cliff tiles, +1 tourism on coast sea tiles.
Oceanographic institute: Unlocks with scientific method. +1 science in every ocean tile, +3 on natural sea wonders and atolls
Tidal powerplant: Unlocks with renewable energy. +1 production in every sea tile adyacent to your city districts
So in essence, a city built on coast rather than inland plus harbour would get far better benefits out of sea reasources in the early game (drydocks, fisheries), better defense on the mid game, just when sea trade becomes important (sea batteries) and increased late game science and production if you really planned ahead your city location.
Love the building suggestions!
Not to mention the whole idea of unstacking cities is that this whole sprawl is supposed to be a city. If a district or districts are on the coast... the city is on the coast. This idea that only the city center is the city seems to clash directly with the key concept that the whole sprawl is the city.
Yeah, the thing that worries people is that Navy won't have many targets to attack/defend. I think just giving incentives to trade by sea is plenty of impetus to build a navy.