Imma be real, it sounds like you had a shitty experience with religion and that sucks. A lot of people have that happen, and it's a significant driver behind the ever increasing atheism of the west. But not everyone has had that experience, and it's not your place to just say "Hey, it's an ethically good thing to just let First Nations religious beliefs, Mandaeism, or Zoroastrianism, for example, die out." It's not just about our knowledge of religious minutia, it's about our understanding of a culture. Religion and culture are deeply, inexorably tied for many people around the world, and you can't just say that your bad experiences with Christianity in a western country are universal and should apply to religions and cultures around the world. Different cultures provide for different perspectives and different experiences, and it would be a true shame to see that disappear more and more over time.

I don't think the Ship of Theseus argument really applies here, and certainly doesn't help your argument. The cells in your body are continually replaced and you are continually changing. Are you not still you despite the fact that all of your cells are routinely replaced? You continue to change, but you are still you.

This withering away overwhelmingly will affect more peripheral and uncommon cultures and religions far more drastically than it will affect those religions and cultures that are more common. It also doesn't result in people leaving religion entirely and becoming an atheist, it far more often results in them converting religions to one that is more popular.

I'm done arguing with you.