2. Tony Stark Has Grown As Much As He Can/Will
The character of Tony Stark was a hard living womanizer, happy to spend his money on women and alcohol while turning a blind eye to his corporation's business functions. Tony blindly trusted those who worked for him to take care of everything, and ultimately ended up getting burned for it when he realized Obadiah Stane was making arms deals with the Ten Rings terrorist cell...using Stark Industries as a front. From that point we saw him try to juggle his personal and super lives, while also facing his dwindling mortality and a foe motivated by his father's past dealings to destroy him. Which brought us to The Avengers, which was the straw that broke the camel's back and causes Tony to now question everything and put his ego more or less on the shelf. It even looks like he's found it in him to settle down and stay with one woman, whom he truly seems to love. Ladies and gentlemen, we've reached Peak Stark. His character has expended so much growth for two solo films and one combo film that there really aren't many places to go from here that don't involve breaking him and putting him back together again. Could we do "Demon In A Bottle"? Yeah, we could...but Disney/Paramount would never dream of it. Could we bring something else in to break and reset Tony? We could, but Extremis, Killian, The Mandarin, and whatever else awaits him is enough. To keep the character in solo rotation would ultimately invite entries that would invoke the law of diminishing returns, and if Marvel allows themselves to do that then they'll fall into the same trap that their direct competition has only recently gotten themselves out of.