Can you image the critique RDJ would get for suddenly coming back as Tony after saying he was done? So many people would claim it's a cashgrab. And I saying this knowing that RDJ recently said he's open to it.
Yes, RDJ coming back would be a cashgrab and utterly shameless on Marvel's part, as it would be acknowledging (to a point) that they need the old popular character back to bolster certain new characters that are not exactly setting the screens (big or at home) on fire.
Oh, and about some tagging Lucas with repeating a plot device, it was one thing for Lucas--the creator--to use a second Death Star, but there's no excuse for others--decades later--to use a "bigger and better" world-killing weapon in TFA, which only added to the impression that the film was a knock-off of ANH (because the PTB could not think of an interesting, creative idea of their own).