Are you sure you watched Empire Strikes Back and Return of the Jedi? Dead Obi Wan not only tells Luke to seek out Yoda in the Degobah System but he also walks around as a force ghost and hangs out with him and yoda and at the end of ROTJ he's back with Yoda AND anakin
Again, those are like, what, two shots in the whole movie? Seriously, go watch the original trilogy again. The roles might have major plot implications, but they're incredibly small roles. Dead Ben doesn't even show up in the first movie (only voiceover).
Anakin will not be in the new movies. And there's no need to recast Luke.