There can be more than one protagonist in a story... In Infinity War...Thanos is the main antagonist... Iron Man is one of many protagonists
Nope, reversed. Infinity War is entirely about the dramatic needs of Thanos. Heck, the filmmakers tell you this at the end, even, when the splash says that Thanos will return. This is an intentional design of this film, and has been talked about both in critical analysis and by the filmmakers. Thanos is the protagonist.
The fact that the emergent story is created from and focused on the decisions the PC's make to deal with this or that threat (introduced by the GM) is what makes them the protagonists of said emergent story. A Threat alons no matter how well detailed is not in and of itself a story.
No, it doesn't, really. To look at Infinity Way again, the choices that Tony Stark makes absolutely create emergent story (within the conceit that this is an emerging story, of course), but he's not the protagonist -- the things he's doing aren't about his dramatic needs, but instead foiling the dramatic needs of Thanos. In a D&D game, this can look like a PC taking actions, and having emergent features occur due to them, when attempting to foil the plot of a necromancer trying to take over the kingdom. The dramatic need in this story is the necromancers -- the game is about his desire to take over the kingdom. The PCs here are reactionary, they do things to thwart this dramatic need, and, if agency is present, they change the resulting story, hopefully to the point of thwarting and defeating the necromancer's dramatic need.
If the players decided to go to that dungeon vs the city of Agrabar is influencing what play is about. If the dramtic need of the PC is to explore dungeons it doesn't matter if the GM creats it or not his dramatic need is being explored.
This is agency.
If the game is focused on the decisions and resulting ramifications of the choices the players are making. They can choose or choose not to make their dramatic needs the focus of play. The only way they lack protagonism is if this option is taken off the table, otherwise they have protagonism and need only exert it.
No, this is agency. For protagonism, the things that are presented must be about the PCs, not just the results of what they decide to do with what's presented.
No I'm not I'm clearly stating what the game is about. You seem to be suggesting that the GM pre-creating something precludes it from being a part of the PC's exploring their dramatic needs and yet I haven't see any evidence to support this conclusion. The crux of this really seems to be floating around how much authorial control is given vs. whether protagonism is present. Why not just say this difference in styles is authorial control then?
Again, if you're making setting details that do not invoke the PCs' dramatic needs, then these are absolutely independent of whatever PC happens to be there, and so are not protagonism. If play largely features things like this, the protagonism is inhibited or not present. The AP I'm running right now features no protagonism at all within it's pages -- nothing about that adventure changes based on what PCs are playing it. The results may change, the path may change, but the AP is otherwise unchanged. This is because the AP allows for agency (albeit slight, it's a railroad), but doesn't allow for protagonism.
Again, I'm very curious as to why this concept is receiving such pushback. It's not a negative to not have protagonism -- lots of great games have little to no protagonism. The very goal of play as stated by
@Emerikol is non-protagonism, but I'll wager dollars to doughnuts that he has a blast in his games. There's no need to have protagonism, it is not a good that improves any game by increasing it's amount or even having it. It's just an approach to games, which most of the people arguing they have it are also strongly in the camp that they would dislike these games. Is it just an argument so that a different style that is assumed to be disliked can't have something that your games do? Very strange, if so, because why else would you dislike those games if not because they have features you dislike?