Holding off doesn't work anymore. It did in the pre-MCU era, but already by the early MCU films it didn't really, and especially now it doesn't connect with audiences, who want a full-fledged story rather than a tease. Today you don't want to see Iron Man fighting Jeff Bridges in a clunky armor to work up to something bigger later, you want to see the hero(s) go up against a very real, unsurmountable challenge on the first outing.