You’re talking about a very, very gradual return to the oceans and at various points. The current process most often means effectively dumping the brine back into the ocean in one place, vastly increasing local salinity. It takes a long time without further brine dumping for that to dissipate and return to normal conditions. In the meantime, a dead zone has been created where the entire food chain is wiped out. It takes a long time for that to recover and it can only begin that long process after the salinity levels have returned to normal. Which never happens so long as brine is continually dumped back to that point.
Imagine a clear lake that’s pretty large, but still small enough to see across. If you dump a bunch of mud in one part, it’s not clear in that part for a while. If you wait long enough, it’ll clear up again. But if you keep dumping more mud there, it’ll just stay that way indefinitely. As time goes on, if you continue this process, you’ll affected a larger area and more severely affect the local area (perhaps changing the shoreline from mud build-up).
The answer here would be to store the brine and combine it with filtered waste water, then distribute it across a larger area where it rapidly dissipates. This is a more costly way of going about it, but you avoid the destruction of the local area.
Why would their be brine? Run the brine through another filtration system, and keep running through another and another until you eventually get salt