"People have been (living) with these characters for a decade and this is the ending". At this time, Joe Russo shared that the actor has not said anything about leaving, but it is only a matter of time because it will have to "happen at some point". He can't play the character forever... I am sure there are lots other things he wants to do in his career. While there are some fans out there who might like to see Robert Downey Jr.in the role forever, Joe Russo also confirmed that eventually Robert Downey Jr. will be calling it quits, echoing comments Downey Jr. himself has made. But he loves playing the character, I know that, and we love working together.
As the AVENGERS INFINITY WAR nears the excitement and buzz around the film goes up a notch as fans all over the world as eagerly waiting to see their superhero on the big screen and know what the future holds as the fate of some of these heroes is uncertain. Other upcoming Marvel Cinematic Universe films include Ant-Man and The Wasp on July 6th, Captain Marvel on March 8, 2019, and the fourth Avengers on May 3, 2019. But new actor or not, Robert Downey Jr. will forever be remembered as Iron Man mainly because he helped to bring the once little-known character to popularity. Thus, it's very much understandable that the folks at Marvel Studios keep their cards as close to their vest as humanly possible when it comes to anything regarding Avengers: Infinity War.
That being said, he did acknowledge that Downey may be replaced at some point along the way, although he seemed to indicate it would be more in the context of a reboot than him simply being replaced a la Edward Norton as Bruce Banner in the early years of the Avengers series.