Avengers: Infinity War and its sequel are going to be very emotional for fans of the MCU. It has essentially been confirmed that Chris Evans will be leaving Marvel after Avengers 4 and many believe that same fate also befalls MCU veterans like Robert Downey Jr and Chris Hemsworth. Whatever happens after the fourth film one thing is clear; no one can replace these actors, particularly Robert Downey Jr, who made Iron Man a household name.
It turns out the Russo Brothers feel the same way, as they also went out of their way to point out how important RDJ has been to the MCU during an interview with Hindustan Times. Joe Russo told the website that no one can replace RDJ and pretty every fan can agree with that statement.
Like everyone else, Joe knows that RDJ won't be playing the character forever and that there are other roles he will be doing after these films. However, the Infinity War co-director has stated that Downey loves playing the character and they love working with him, which is pretty vague, to say the least.
"He can't play the character forever. I am sure there are lots other things he wants to do in his career. But he loves playing the character, I know that, and we love working together. We will see where it goes from here."
That doesn't really answer anything about his post-Avengers 4 status but it seems like the only one who knows if he's sticking around is RDJ himself. We will see if he plans on resigning with Marvel or leaves permanently but it won't be the same without him.
Avengers: Infinity War comes out on April 27, while Avengers 4 comes out on May 3, 2019.