[simonh]

No, for several reasons. You might be sure that the value will fall but not know when. There might be short term risks that might make now a sub-optimal time to make that bet. There might be other effects that might mitigate this particular effect - so you might be convinced that Brexit will drive down the pound compared the the value it might otherwise have had, but not know what that price driven by other factors might be. Government intervention might mitigate the effect - George Soros made a fortune betting against the government but many others have lost fortunes the same way. The trading floors eat naive market perfectionists for breakfast.