I think there's already enough Williams Star Wars (and other scores) out there to form the basis of future soundtracks. There's a lot of music on Star Wars videogames that has the Williams feel and quote his themes. We don't have to worry about when Williams retires - a decent composer will still be able to follow in his footsteps well enough, he's done enough groundwork.
It's still good to have the real thing for the new movie though...
Agreed on all counts.
I don't think there's any reason that Williams needs to have an "understudy" working on the new film. First off, yes, he's 81, but he's in good health, still very active, and still has a thriving career. For all we know, he could be scoring movies for another 20 years.
Second, as you said, it's not as though other composers aren't familiar with either the existing Star Wars
material or Williams' style. A decent composer could continue in that tradition if need be. They wouldn't have to be specially "prepped" for the job.