That sounds like a recipe to completely destroy the land, for _any_ other crops, on which saltwater crops are farmed.

Even if we aren't irrigating crops directly with seawater, we may have to engineer crops to thrive on high saline irrigation anyways, as there is seawater leeching into aquifers in many agricultural centers around the world, including coastal California. As sea levels rise, we should expect this to be a larger problem and plan for it.

Assuming you would use salt water on existing land, but wouldn’t it open up for growing crops in new places, on/close to seawater?

If you want to grow crops on the sands of the beach, maybe. Otherwise you're poisoning whatever land you're using.

That’s not feasible over time. Evaporation increases the salt content of any irrigated soil over time. Directly using sea water would slowly cover your soil with a thick layer of salt.

To get around this you would need to keep the area underwater and slowly replace your water with sea water. But, at that point you’re better off just farming the ocean which already covers the majority of the earth.