the numbers don't work

The solution is real or clearly buildable, and the economics stop it. Desalination works but costs too much for the people who need the water. One special case is worth knowing. A design choice made upstream for good product reasons, such as stretchy fibers in clothing or glued-in batteries, creates a heavy cost downstream for whoever has to recycle the product. That party has no way to send the bill back to the designer, who has no reason to change. The fix is usually a new business model, price signal, or policy channel.