the idea that the best narrative is the one that thrives

I was pretty clear that the effectiveness of a narrative is dependant on the results you're seeking. I think you can turn a narrative loose into the world and it will run autonomously to a degree, and you could use a story's ability to thrive and survive as a measure of at least how attractive and engaging it is, but no, I don't think that is what makes a story effective for the purposes of influencing a large amount of people to make better choices, to have more curiosity, to think more about things they don't normally think about.

Social engineering like this does take deliberate work. It takes effort and work to keep a story alive and growing. The problem is we already have tons of people doing this work for their own agendas. Sometimes they're good stories, sometimes they're terrible stories, but it almost doesn't matter the "quality" of the narrative, since our brains are designed to hook into narratives to explain the world even if the explanation doesn't even make a lick of sense. See: anti-vax doctors and flat earthers.