When did a backstory become a warranty? It establishes your character’s past, but some players expect it to secure their future too. Their "heroic destiny" becomes something the DM is supposed to deliver. If keeping you alive until you get a satisfying ending is part of the deal, I call it plot armor.
A campaign arc gives the group a shared goal. It doesn’t guarantee any individual character’s destiny. I want us to have something worth pursuing, with our decisions determining whether we get there and what it costs us. Having a stake in the outcome doesn’t mean my character is entitled to survive it.
Expectations also change how players approach danger, which is where combat as sport comes in. It’s probably my biggest issue with modern D&D. Once players assume every fight is suitable for their characters, “should we fight?” starts giving way to “how do we win?” They’re choosing tactics instead of deciding whether those monsters are worth tangling with. That changes the whole game before anyone rolls initiative.
A fight designed to be winnable doesn’t automatically mean the DM will prevent you from losing. But pair that expectation of suitable fights with a heroic destiny the DM is supposed to protect, and how dangerous is this adventure supposed to be? It’s Avengers with swords. Everyone gets to be a superhero, and the DM gets the job of making sure it works out.
When the dice or the players’ decisions threaten that promised future, the DM steps in with a deus ex machina. Someone arrives just in time, and everyone lives to fulfill their destiny. Keep doing that, and players learn that getting in over their heads is the DM’s problem to solve. Sure, plot armor has always existed, but I object to being expected to provide it.
I want a traditional fantasy adventure with actual stakes. If you guarantee everyone survives to fulfill their “heroic destiny,” then you’re playing a cozy story game with dice. Putting a dungeon or a dragon in it doesn’t make it dangerous.