The salt comes from the ocean in the first place, and the water returns to the ocean relativly quickly. Unless something changes and we start to notably change the volume of water in the ocean, dilution is the only way to maintain the ocean chemistry. If we don't, then we would be reducing the salinity as the water returns without the salt.
The problem is that it isn't perfectly diluting, it's concentrating the salt in areas near the desalination plants, changing ecosystems and causing problems locally.
There's also the semi-related problem of salt that isn't from the ocean making it's way there, increasing the overall salt concentrations over time.
If we could find a way to use the salt from desalination (at least partially) somewhere else, then maybe we could help offset the "saltification" of the ocean and can more easily avoid destroying ecosystems around desalination plants. (it doesn't need to be an all-or-nothing solution, we could still dump some salt back into the ocean while keeping some out and used in other areas)