Metals however, are often the most recyclable materials we use, because unlike carbon-based materials like plastics, metals often have useful properties in their elemental state, or as alloys that can be melted down and reused without the loss of those properties or of much material.
Most aluminum, for example is recycled, because the cost of recycling it is lower than the cost of mining new material.
Also, those intakes for water are going to be massive. How are we going to make them fish safe? Dolphin safe? Plankton safe? This is a major problem in hydroelectric dams.
Third, what's providing power for this process and where is it located?
Fourth, what are the second order effects of replacing a lot of steel production? Will this make all of the remaining products where steel can't be replaced a lot more expensive? I doubt you can use magnesium as a replacement for the girders used in buildings. Magnesium is only as strong as mild steel so pretty much everything that requires any tensile strength will still need to use steel.
Okay, so California ends its drought permanently and becomes a major supplier of magnesium and table salt.
Slightly-less-facetiously: sodium chloride has all sorts of industrial uses (on top of its culinary uses). The good uses are, in other words, already thought up in droves.
Further, most salt production already entails taking seawater and pulling out the water. Ain't too much of a stretch to, you know, actually keep the water instead of letting it evaporate into the atmosphere.
So, no. This is not a serious concern.
In absolute terms, we couldn't hope to remove enough salt from the oceans to be even so much as detectable in the absolute sense, but in local terms water with increased salinity can cause problems. Oceans are not hives of life everywhere you look, it's really just in spots, and those spots are generally right where we want to be and to put our desalization plants.
Plus the high-salt and normal ocean water are much more resistant to mixing than our intuition would suggest. They will eventually mix, but the high-salt water can go a surprisingly long way first.
Not going to be an issue with your average desalination plant, but certainly proves the point that water masses can behave differently in big, non-trivial ways due to their salinity.
I'm unsure on the impact of costs. Maybe we just pump it into the salton sea....
Every time there's a natural resource humans want to exploit, this is almost always been an argument, and it's almost always been wrong.
"look how many fishes are in the river! You can walk across the river by standing on the fishes!"
"Look how trees are in this forest! You can't see the end!"
Not to get all Derrick Jensen on this, but a red flag has been raised in my mind, as has my left eyebrow.