As referenced in both RDJ’s words and his lines in Age of Ultron, Tony Stark knows and is fearful of a more aggressive threat than the one that opened a wormhole in New York City in the 2012 film, and he doesn’t know how they can be prepared. He tried to create something that can replace the heroes so they don’t have to come against it, but it too fails, leaving things to the Avengers.
Article source: https://www.cinemablend.com/news/2464582/heres-robert-downey-jr-teasing-endgame-during-press-for-avengers-age-of-ultron