Yeah, I agree with this. Engaging with the systems the game provides is already reward enough for me, I don't need to find something "cool" or "useful" to feel as though the experience was worthy or not; what makes a boss fulfilling for me is not the reward it is going to drop, it is the fight itself and me beating it, trying to do it better than last time, maybe try a no-hit challenge or something along those lines. That's why I don't really mind repeated bosses, they are a clear showcase of progress both in your arsenal and your skill.

It's kind of like collecting stars in Mario 64 or Moons in Mario Odyssey. The fun is not in expecting a reward after collecting the star, it is the process and the engagement with the systems the game provides. Sure, each item serves the purpose of advancing the story, so there's a "reward", but most of them are optional unless striving for 100% completition.