the WAL is not for when your disk dies. It's for when the server crashes. First writing what you're going to do to the WAL before doing it makes it possible to recover from a crash at any point. If it crashes during writing to the WAL? Ignore the half-written WAL records and keep trucking. If it crashes during writing to the data file? Use the WAL to rerun the writes that didn't complete.

It doesn't guarantee you don't lose data written during the crash, but it does guarantee you can get the database back into a usable state.