Obi-Wan wasn’t strong enough to win and his job was to keep an eye on the boy. If he hadn’t gone into hiding, Vader and The Emperor would have surely killed him and who knows what would have happened to Luke. Certainly he would have never received the droids or had a reason to go to Alderaan.
I felt like at the end of the OT, Luke was the strongest force user in the galaxy and unassailable by whatever was left of the Empire.