Apparently they don't need to worry about RDJ not signing up for a 4th film, since they used a body double and digital replacements in IM3 most of the time anyways.
Well, you still have to pay RDJ for the voice.
Then it's just a question of which is cheaper: paying RDJ to be on-set, or paying the VFX vendors for all that work.
Since VFX people are exploited like slaves and happy with it, it's cheaper.
The most interesting thing about the article is that the schedule of summer blockbusters is so extremely tight that they don't even account for possible injuries of their main actors. Which is incredibly stupid considering that their main actors are doing stunts themselves. And then they end up torturing the VFX houses, who comply with everything because otherwise they go bankrupt. But then again most of them also go bankrupt when the film is a success.