Chris Evans has been flaky on if he'll still be the character after Infinity War...but honestly, I think that sometimes the actors will start spreading rumors like that to up the prices on their contracts. Marvel movies are guaranteed, high paying work, almost every year, and they are highly reviewed blockbuster movies at that. You have to remember that money is not always guaranteed with normal acting, even if you used to be good and have just fallen off the map (George Clooney, Michael Keaton, etc.)
That's why Paul Bettany was so excited to be the Vision - he actually had just got off a call with his agent that told him his career was over, and then got a call from Joss Whedon to be an actor in a high paying, multi-contracted role.
But the actors know that they also have Marvel on the ropes too, because they're so ingrained with the character that Marvel just can't recast Captain American or Iron Man like that. So they spread these rumors and then play the game to get a better contract.
That is definitely why RDJ has not resigned his contract. He knows that if they really want Iron Man, they'll pay for Iron Man. And having rumors going around only helps to up that paycheck.
(For the record, while I don't know if Cap will be a sidekick in Black Panther, I do think that the Black Panther will give Cap a Nomad shield made out of Vibranium. Thanos will then come, wreck the Avengers and break Cap's shield in half...and then Iron Man will give Cap his old shield back right before the final battle.)