yep, that is probably the top reason, they will avoid those storylines or Episode VII will take place many years after those storylines. I am way behind on my reading, I think the books are about 40 years past Episode VI now, so that would age Ford, Hamill and Fisher about right to jump over all the books storylines.
First off, there's no way there would be an Ep7 without the original three. However, am surprised there's no love for Lando.
Someone mentioned above that Hamil didnt want anything to do w/ Star Wars.. Sorry mate your wrong. Hamill has been saying for decades now that he was coming back as a "Obi-Wan type character". BD Williams is the one who's mostly annoyed w/ Star Wars, however money talks so he'll be back too.
When Peter Mayhew signed on for Ep3, there was a clause in his contract that he'd be reprise the role for Ep7,8 and 9.