As for the likelihood of Ford being relegated to a "bookend", has there been any real basis for expecting such a scenario to occur? I realize fans have discussed it for a long time- presumably because they couldn't imagine Ford playing this role in his seventies- but it doesn't appear to be a sentiment shared by anyone involved in the actual production.
Frank Marshall's comments about no one else being cast in the role seemed to rule out the possibility of seeing a younger Indy via flashback. Spielberg has essentially echoed that intent and has long said he can't imagine anyone else in the role. Even Disney was said to be very pleased internally with Ford's involvement in Force Awakens, and there's no doubt they're hoping to duplicate that same successful formula with Indy.
It would be different if a new creative team was stepping behind the camera with "reinvention" in mind, but we're talking about the same core group that loves this franchise and all its traditions. I'd say we're far more likely to get an older Indy aided by a comic sidekick (Chris Pratt) and ass-kicking heroine (Daisy Ridley) than a variation on "Mystery of the Blues".