I dont think you would like what comes after societal collapse. It's easy to pin society as just capitalism, but collapse will mean more than just the economic system. Democracies will collapse and entire regions will cease to exist. Food scarcity and mass migration will result in extreme regimes that will defend their territory, and a bunch of nomads who have to live with the constant worry of where the next food and freshwater source is. Not to mention the constant fighting over geopolitical issues (imagine current day scaled up exponentially)
Yes, we should fix our economic system, but societal collapse is not an end result we ever want.
Salinity doesn't really work like that. You can't just dump a bunch of brine and expect it to just mix with the rest of the seawater. A lot of that depends on temperature, currents, etc. You might just end up forming a brine lake in the ocean if the brine just so happens to end up at the bottom without ever mixing. Not to mention brine isn't always just concentrated salt and water. It can include byproducts from desalination.
Reference:
https://www.scientificamerican.com/article/slaking-the-worlds-thirst-with-seawater-dumps-toxic-brine-in-oceans/