Are you sure the math even remotely checks out? Just looking up some numbers, even if we naively assume ocean "saltwater" is just H2O + NaCl (which it obviously isn't): the world consumes around 300 billion kg salt/year, whereas (say) Los Angeles alone uses 700 billion kg water/year. So even if a city like LA provides for the whole world's use of salt, what does everyone else do?
> On average, about 35 g of salt is present in each 1 kg of seawater
so if those 700 billion kg of water for LA all came from desalination, that would yield 24.5 billion kg of salt.
That suggests we could have 12 LA-sized cities relying entirely on desalination before we'd saturate the world's demand for salt.
"Could have" is doing a lot of heavy lifting there. Assuming spherical cows, 12 is an upper limit, okay. What you could actually do is neither remotely close to that nor is that itself remotely close to what the world needs, was the point.
Adding your 12 cities would double the supply.