It’s a real testament to these actors’ talent that they could create such iconic characters. As a result, audiences can’t help but constantly be reminded of the characters when they see the actor's face. When fans unanimously agree that no other actor could’ve played these characters, it’s safe to say the casting director made the right choice.
In hindsight, so much was riding on the first Iron Man film to be a success, so the MCU got a good start with Robert Downey Jr.'s fan favorite performance. The movie famously revived Downey’s career and launched a film franchise that’s reached inconceivable levels of success. He has since become one of Hollywood’s highest-paid actors and will be immortalized in cinema history as one of the greatest superhero performances of all time.