One way to see the issue is like this:

Potable water is water that has been separated from all the junk it was dissolving. Due to entropy, this is energy-wise expensive; so instead of doing it by ourselves, we humans have been relying on water that has been purified by natural processes (evaporation, then rain, then rivers). That water is cheaper because we don’t need to pay for the energy costs associated with it.

However since we’ve been using far more of that naturally potable water than nature produces, the demand for that water is higher, so its price increases. The hard cap is the price of artificial potable water. It’ll never be as expensive as water purified by human beings, because once it’s too expensive people simply shift from one to another.

That shows another advantage of investing on desalination: cheaper desalinated water prices means that price for water in general gets cheaper.

Pumping this supersalty water back into the ocean can harm local aquatic life. Reducing these impacts is possible, but it adds to the costs.

Why not process it further? Perhaps the amount of impurities is too large to use this brine for table salt, but you can electrolyse it into sodium hydroxide, hydrogen, and chlorine. Those are chemicals that the industry uses a lot of.

We could also just pump the excess water into an evaporation pond and let it naturally evaporate. We’d be creating salt stacks but static salts better than liquid salt.