Marvel has redefined how Hollywood makes movies. Not saying sequelitis or forced franchises didn't exist before, nor that they were the first ones to try to do the whole connected universe thing (the Universal monsters beat them by decades), but that the proliferation of the Cinematic Universe in the modern era is definitely Marvel's doing. We have of course the DCEU, but also the Godzilla/King Kong MonsterVerse, Universal's ill-fated DARK UNIVERSE, the CONJURING-verse, and the quickly buried AMAZING SPIDER-VERSE – among others. And none of that would've been possible without Robert Downey Jr.
See, Marvel for a long time wasn't doing too hot in the cinemas. Even when they wanted to start making their own films around the mid-2000s, they had already sold off the rights to their most cherished characters (ala X-MEN, SPIDER-MAN, and the FANTASTIC FOUR), so they were already starting from a point of weakness. Not only that, but by the time they made Iron Man he wasn't very popular (as he was essentially the villain of the comic version of CIVIL WAR that came out around that time). But Marvel had an ace in the hole that made this whole crazy shared universe thing palatable – the aforementioned Downey, Jr.
However, unlike comic-book characters who can be 35 for four decades, human actors can't pull that off. So it's only inevitable that at some point he will have to step down. And here's what he had to say to the Australian site News.com.au:
It’s this cyclical thing…I could have said when the first ‘Avengers’ came out: ‘It’s never going to get any better than that. Everybody stop’. But to me it’s always about people and opportunities, like the Russos, who I adore. Everyone says to me it’s like a glove that fits so well. I have to start over every time but I am starting over with a pretty solid base. I just never want to blow it for the last six or seven [MCU movies] I have done by dropping the ball because I decided to go do it one more time. I just want to hang up my jersey before it’s embarrassing.
Downey, Jr.'s initial contract actually expired after AGE OF ULTRON, which had to be re-negotiated to reappear in CIVIL WAR and subsequent films. So to hear him say that he'll be leaving soon isn't surprising (I'm sure Feige doesn't want to keep re-negotiating either). Not only that, but Feige also said that INFINITY WAR could mean the end to a few of the Phase One characters, and with Chris Evans making it clear INFINITY WAR 2 is his last film, Robert Downey, Jr.'s Tony Stark/Iron could be one of the other casualties. I guess we'll see when the film comes out.
Meanwhile, you can next see Robert Downey, Jr. as Tony Stark in SPIDER-MAN: HOMECOMING, swinging into theaters July 7th.