Except for the fact that we know damn well Tony *does* create new suits for AoU, including Hulkbuster. Yes, the "I am Iron Man" in IM3 quote is definitely metaphorical
Tony feels the need to protect the world again as an Avenger. That's kind of the whole point of Avengers 2, and of Phase 2 in general --- the heroes are coming to realize that they can't go around saving the world on their own anymore, and that teaming up to form an official Avengers is the best way they can go about protecting humanity from these larger-than-life threats.
Tony doesn't need to build a killer robot to "save" the world. He doesn't need to reboot the Iron Legion to do that, either. He just needs his super friends.