I don't even think you need to kill off superheros, you just need to give them space to lose or get pyrrhic victories. All of Captain America's movies are great at this: Cap always gets a win (at least compared to the alternative playing out) but at a huge cost to himself or a large impact to the greater storyline.

If you know that the good guys are always gonna win with no serious consequences, then it makes you wonder why you should bother watching the same tropes play out over and over. I feel like a lot of the latest Marvel series/movies are suffering from this: not necessarily because the whole no consequences thing is new (because it's a valid criticism for some of the older marvel flicks) but because we're tired of it and we need a reason to watch something new.