The ocean is way too big for removing some drinking water (which is going to end up back in the ocean anyway) to have a material effect on the salt concentration.

The problem is strictly local: if the brine produced by the plant is dumped in an area with low flow, it can hang around and affect sea life.

If you use a big discharge pipe and put it a decent distance offshore in a current where it can mix effectively, there’s very little effect.

This is not a reason to shoot down desalination plants as a concept, it’s a design constraint to take into account when building them.