The whole back story is dumb. If Obi-Wan had any serious plans with Luke, he would have raised him himself and, more importantly, started his training at the very early age.
I have two explanations for this thing. One is simply being a plot hole. The other is that Obi-Wan and Yoda knew that Luke could never defeat the Emperor and therefore they planned to use Luke just to trigger Anakin's return who would then destroy the Emperor (which is what eventually happened in ROTJ).