Tom Cruise was genuinely attached to the idea of playing Tony Stark. According to the official book The Reign of Marvel Studios and Kevin Feige himself, when the rights sat at 20th Century Fox, a then-34-year-old Cruise flirted with the role, but his asking fee at the time was simply more than even a profitable studio was willing to risk on an untested superhero property. That is the real reason it fell apart, less a dramatic clash over creative control and more a matter of money on a project nobody yet believed in.
Cruise himself has downplayed how close it ever got. Speaking to ComicBook.com in 2018, he said he was not close to taking the role, adding that he loves Robert Downey Jr. and cannot imagine anyone else playing Tony Stark. When Marvel Studios regained the rights and set up its own scrappy operation under Feige, they could not afford a star of Cruise's level anyway, but they could afford Downey Jr., who at the time was still shaking off a troubled reputation and was considered a genuine casting risk. Director Jon Favreau fought hard to convince studio brass to take the chance on him.
That gamble reshaped Hollywood. Iron Man became a surprise hit in 2008, launching a franchise that has now earned over 30 billion dollars worldwide, and Downey Jr.'s Tony Stark became the emotional backbone of the entire Infinity Saga. Funny how the version that almost happened rarely turns out better than the one we got. Can you even picture Tom Cruise as Iron Man, or is RDJ just too perfect to replace?