I respectively disagree, from what the video shows it's just a bunch of killing tasks a maze and an agility course, there's no story, no depth, no charm or character; players do the thing to get the reward. Yet I won't deny that it was well executed, the theme was consistent, even if I don't see the reason for it other than "fire + lava = cool".
I do agree that new rsps players will fluctuate based on osrs popularity, but it's not a zero-sum game, players can play both at once. The reason they don't for long is that the content lifetime of a server is a month or two. RSPS's hook has always been short-term gratification, but server's struggle to keep the majority of players because they make content goal-oriented rather than journey focused. You new minigame is a perfect example of this, I bet players are doing it #1 because it's the new thing #2 to get the reward. After most players have the rewards I expect it'll be dead content because the content itself isn't engaging.
You more than most will have the data, I'd love to see some numbers on the content and player churn rates.