It was all planned out ahead of time. Once they are under way derailing the MCU requires Iron Man to tank hard—totally plausible, given that it didn't have a script and Marvel were unable/unwilling to delay the film given their contract commitments. The only reason they made Iron Man was because they were setting the whole thing up for Avengers after all, so therefore the only way to stop that is to have Iron Man implode as a movie. Just skipping the end credits does nothing.