I’d disagree about that. Tony Stark’s arc over the entirety of the MCU led to exactly that result and his actions in Civil War were all directly influenced from previous movies.
Iron Man: Tony has his revelation in the desert and when Ho Yinsen dies as a result of his weaponry. Decides to build a suit of armor to personally fix the problem
Iron Man 2: Tony believes he’s terminally ill, sets up Rhodey to take armor, refuses to tell people or ask for help, SHIELD determines him too erratic to use
Avengers: Tony grabs a nuclear weapon and flies it through a wormhole in a potential suicide run to save NYC after multiple life/death fights
Iron Man 3 A PTSD suffering Tony gets into a personal fight with a terrorist who bombs innocents and destroys his home, undermining his sense of security, uses dozens of iron men suits to fight them
Avengers 2: Tony has built a whole drone army essentially, then has his own deep fears and lingering PTSD (again, he is TERRIFIED of failing people and being responsible for their deaths) used to direct him to end up creating Ultron, who is himself an extension of that desire to PERSONALLY SAVE EVERYONE. Not to mention building and personally piloting an anti-Hulk weapon; Ultron and his drones ultimately kill a lot of people
Civil War is basically the culmination of several films leading Tony to the place where he reasonably says, “We need to be supervised, because my technology keeps killing people and when i try to use it to fix things, it often kills more people.” Repeatedly, Tony makes things and half the time, they get used in ways he doesn’t want or intend. He feels extremely guilty about it, is suffering from PTSD and has figured out that every time he tries to be the smartest guy in the room, it backfires on him.
Steve, meanwhile, has had a history of going AGAINST authority on moral grounds. In CA: they want him to be a propaganda tool, until he goes AWOL to rescue the Howling Commandos. In CA2: he goes AWOL when he realizes SHIELD has been infiltrated…but he wasn’t on board with Fury’s initiative before it went south. In Avengers, Steve sees first-hand that the council behind SHIELD would willingly nuke New York and isn’t cool with it. In A2 we see that everyone looks to Steve as the moral center.
One thing I greatly appreciated about CA:Civil War was that it was much more logical than the comic series, which had Tony Stark represent the registration side because a lady said mean things to him (after a villain blew up a school) even though he had nothing to do with it. It made even less sense because in the comics, Tony is often more likely than Cap to go against the government when his morals tell him to (or at least FASTER than Cap does, as Cap does it more often, iirc). In the comics, Tony attacked SHIELD agents when he thought they were using his tech, for example (see: Armor Wars, Armor Wars II, Extremis, etc.)
I mean, if you thought the plot had issues, I’d agree with you there…but the movie version of Tony was, IMHO, acting in line with his behaviors up to that point.
Sorry, I did go on there, didn’t I?