There's a lot of debate on how effective Obi-Wan's plan would have been if he hadn't had help from the Force, but there's no doubt that he was clever enough to come up with a backup plan. It's possible that he knew the Death Star could be destroyed if someone flew into the reactor core, but he couldn't rely on such a risky strategy without knowing for sure. So his backup plan was to delay Darth Vader and buy Luke time to destroy the reactor. Obi-Wan was one of the most powerful Jedi in history, so it's likely that he knew more than he let on about...