I agree it's difficult to see how it could flop, since Iron Man was considered (and still is, I think) one of the best superhero movies to date. You'd have to change the production of the film, like different actors or director.
But assuming it did flop, however it happens, the entire Marvel film project is strangled in its cradle. IOTL, the success and goodwill of Iron Man is what allowed Marvel to press on with their ambitious cinematic universe, even after Hulk underperformed and Thor and Captain America didn't capture anyone's imaginations. But here, without even Iron Man to lean on? The MCU never happens, and that has some massive ramifications:
• Marvel never becomes the cultural juggernaut it currently is. Characters like Captain America or Thor remain B-listers; characters like the Guardians of the Galaxy remain obscure.
• The idea of a Cinematic Universe is never tried. This means that every other studio doesn't suddenly scramble to make their own, with interesting ramifications of its own. For instance, whenever WB gets around to making another series of Superman movies, it will be just a series of Superman movies instead of the sequel transforming into some crossover showdown and set-up for other movies. Sony doesn't overextend themselves by trying to create a Spider-Man universe; Fox probably just lets the Fantastic Four rights expire; etc.
• Regardless of if he's in Iron Man or not, Downey Jr.'s career goes in a completely different direction. Instead of a sudden rebirth as a funny, jokey action star, he probably sticks to dramatic performances in small movies and very gradually rebuilds his reputation