It's not only the WAR that you're trading, but the goal is to trade for cost controlled players, where the saved money could go a long way in improving the team in other ways.
Of course there is no certainty, but hopefully you trade for some youngsters who will become the next Mookie, or somewhere close to it.
IMO, if we know we're not extending him, it would not be very wise to let him walk for just a draft pick, especially when there would still be a possibility of re-signing him as a free agent.