Honestly and take this in the right spirit. You are using a word that doesn't mean what you think it means. I understand your idea and how it works. You call it protagonism. I get that. But protagonism in plain english does not mean what you think it means which is why everyone is opposing you. They aren't arguing the concept. They are arguing the term.
You said:
If I create a world that doesn't care about the PCs and then turn them loose in it to see what they uncover/do, is the story actually about those PCs?
My answer:
Absolutely it is about the PCs. Yes it is about the PCs. As you have reminded me so often, what happens off camera is not central to the players playing of their characters. The real action is with the group. They are absolutely protagonists in their story.
What you've done is add a bunch of qualifiers around protagonism that are not at all required. You needed a name for your game constructs and chose protagonism. Fine. Just realize that the raw meaning of the word is absolutely not the definition of that game construct. If it's gamer speak shorthand that is fine but realize you may need to explain yourself. You redefine a word and then get upset everyone balks at it. You don't get to redefine english.
At this point if you disagree I don't see much further point to continuing the discussion. We just don't agree with you. So your use of that word is a stumbling block to the discussion that could be had about your actual style versus word definitions.