who's to say episode 7 8 9 will necessarily follow the events of Return of the Jedi? aside from the sequential episode numbers, surely the 20 years between episodes 3 & 4 would be a much more interesting era to mine than watching middle-aged Luke, Leia and Han (or even worse re-cast Luke, Leia & Han) running around the galaxy with their annoying offspring.
The rise of the Empire, Jedi purge and the birth of the rebellion definitely is a much richer time-frame to explore, and if they required actors from the prequels they'd still be the right age for the role, and importantly all 9 films would chronicle Vader's rise and fall.