First, it should be clear that it's possible for a story to have tension even when the readers know that our protagonists will succeed and be fine. Most obviously, you can reread a book, and still feel the tension, even if you know with a hundred percent certainty how it will come out. You can read a book in a continuing series -- James Bond, say -- and be reasonably certain that whatever happens, Bond is not going to die, since there are fifteen more books to come. You can read a book in a genre, like most children's fiction, where some kind of happy ending is pretty much guaranteed, but in a well-written work it doesn't remove the tension. That this is possible should be obvious by the fact that books and movies have been very successful in all those situations.

I think of this as the "rollercoaster effect" -- like a rollercoaster, where there's no real danger but you get excited or scared anyway -- but it's really just a part of the suspension of disbelief. The reader suspends disbelief in the world of the narrative, buying into the premise and the characters, and at the same time at some level agrees to temporarily forget about the meta-story stuff, based on their knowledge of factors outside the story itself -- that this is book one of a trilogy, for example.

When readers start giving the critiques I mentioned -- that characters feel like they have "plot armor" -- what has really happened is that suspension of disbelief has been lost. It's not the fact that meta-story logic leads gives information about the outcome that's the problem, but rather that people are thinking in meta-story terms at all. If the readers are analyzing your story in the context of its place in a multimedia franchise, you've already lost them; they should be caught up in it, willingly going along with your characters and world. This is why adding more death, essentially another meta-story option, doesn't really help -- you may be able to convince readers that anyone can die, but you haven't fixed the suspension of disbelief problem that got them there in the first place.