There may be an element of Ford wanting to be involved - if not as the lead (which is clearly his preference), at least in some way. Not that I expect him to pass away imminently, but the guy's gonna get to the point where he's not going to want to be wandering around movie sets.
If nothing else, there's the "flashback" conceit, where old Indy is recalling young Indy's exploits. Done blandly, this is just fan service, but there are some interesting things that can be done with it - putting old Indy in a story where the past is relevant to the present, or the like.