I don't see why an Indy 5 would be so bad. It's not like anyone would expect him to go under a truck again. The stunts can be believable if done right, and still make for an exciting film. It doesn't have to be a slow, actionless film with Indy in the backseat; Just make it believable (No more crazy stuff like Cutting Bridges or getting in Fridges); Maybe amp up the gunplay in place of physical stunts. Indy doesn't have to be 70 just because Harrison is; Harrison with makeup can still pass for an Indy in his late '50s or early 60s like in KOTCS. The film could still be set in 1959 or so. A fit 50something guy can still be pretty tough; it's not an ancient age. A stunt actor might have to be used, but then again, most of the later action scenes in ToD were done with a stuntman in Harry's place due to his back injury.
It doesn't have to be as action oriented as Raiders or TOD IF the story holds up.
Consider that Anthony Hopkins portrayed a 60 something year old Zorro in Mask of Zorro and still was able to pull off believable actions and still came off as a badass despite his older age. I'm not talking about his role in the film as a Mentor but strictly how he portrayed an older Zorro.
Make Indy's action more clever, perhaps, less two fisted. Bring back the gunplay of Raiders. It is possible; It can be done. Harrison still has an Indy film left in him. I'd rather have Indy go out with a bang or disappear into the mists than see him settling into quiet, boring, married life. For a hero like Indy, boring married life is kind of a fate worse than death.