I agree that George Lucas probably didn’t think that through carefully enough.
We know if you want a decent Jedi, you should start the training really young. So, it seems Yoda & Obi-wan weren’t intending on training either Luke or Leia. The only reason Obi-wan came out of retirement was when Luke showed up with a message from his sister, which seemed to indicate she was in Vader’s clutches, which was the last thing they wanted.
It kind of seems like the plan might simply have been to let the Jedi die out, and hopefully the Sith would die out with them.
It apparently didn’t bother the Emperor that he & Vader were the last of the force users, Jedi or Sith. Just as long as the Emperor got his revenge on the Jedi.
Now after the events of episode IV, when Yoda was finally convinced to train Luke, I think it’s quite possible the plan was to fully train Luke into a Jedi Master, and have him take out both Vader and the Emperor.
Was it ever stated in the movies what Luke’s potential power with the Force was? Who’s to say, if after Luke confronted Vader and “completed his training” that he couldn’t have gone on to confront the Emperor in a few more years? We know a powerful, well trained Jedi can deflect Force Lightning.
The greatest danger may simply have been that until Luke was fully mature, the Emperor might turn him. He might have some nasty lightning bolts, but he’s never been able to turn a mature Jedi Master.
Oh wait, there was Dooku. But Dooku kinda converted himself, didn’t he?