Michael Jai White is in FAR better shape and looks far younger than Robert Downey Jr..and RDJ was still in his 40s when the franchise started so I don't see what u gettin at..
Look, there is a big difference between early 40s and late 40s. Like i said RDJ is 48 now, and since turning 48, he's done 4 movies. You want to start a franchise with a guy who is 46 now... so if they got a script together and started pre production today, by the time the movie is released, White would just be turning 48. And that's just one movie.
Studios want to start a franchise with young guys to build a franchise with as young as possible. It's just that simple. Thats why typically whenever studios sign on an actor to do a comic book movie, they do multi-picture deals.