That to store enough energy with just haline gradient the reservoirs need to be enormous

And under water construction is expensive. And durable construction in a marine environment is challenging (and makes things more expensive).

That doesn't mean it's a bad idea but they are factors that add to the overall cost. 20$/kwh is very attractive of course. But that's also a number that e.g. CATL is chasing with sodium ion batteries. And they are going to be making those by the gwh/year from next month.