From a Hollywood Reporter article on Star Wars 7:
Arndt is said to have focused on the offspring of Luke Skywalker (Mark Hamill), Han Solo (Harrison Ford) and Princess Leia (Carrie Fisher), with the original trilogy’s heroes taking on supporting roles. Abrams, however, wanted Episode VII to focus on the classic trio of characters, so audiences could have one more chance to enjoy them before a fitting send-off. The new characters, the offspring, will now be in supporting roles, according to these sources, and take center stage in Episode VIII and IX. Some characters have disappeared from the Arndt script and new ones are being added.
Star Wars creator George Lucas, who directed the 1977 original and each of the most recent trilogy, was also involved in the debate, according to insiders. But Lucas, who sold his Lucasfilm to Disney in 2012, acquiesced to Abrams and producer Kathleen Kennedy, who now runs Lucasfilm.
Look, I’m all for Lucas stepping aside so that the next generation of filmmakers can take over Star Wars from him, but I think he’s right in this case. His plot outlines were never the problem, really. The end of Return of the Jedi is closure times 11; there is no need for a more fitting send-off than that. So let the old characters be Obi-Wans to the new ones.