Earlier quoted context omitted.
Most desalination plants do just pump the brine back into the ocean. That isn't ideal from a marine conservation standpoint but if the outflow pipe runs out deep enough then it's not too bad.
If it's not ideal right now, how well does it scale if more and more desalination plants come online to meet freshwater demands, and dispose of their brine in the same manner?
The problem is the criminal mispricing of water in most of the world. Anything that interacts with saltwater - especially brine - is in a completely different class of infrastructure that requires constant maintenance, something that most political systems are especially bad at. It's compounded by the artificially low price of water and accompanying lack of funds.
Humans have built over three million kilometers of natural gas and oil pipelines globally. The longest undersea pipeline is over 1,200 km long, which is at least twenty times longer than the longest pipelines we'd need to build for ideal brine dispersion in the worst case geographies. For better or worse, the problems with desalination are economic and political, not technological. We just need to spread out our impact so that it's within the margin of error of ocean currents and evaporation.