See and that's where both sides are talking past each other.
For some people it is about the dungeon/campaign world and characters are merely vessels for exploring that dungeon/campaign world. If one character dies or becomes too hopelessly gimped to play, you roll up a new character and continue on.
For other people, it is about the story of a particular group of characters. In such a case, random rolls leading to death and/or uselessness are disruptive to that character arc. If a character dies, all of the character specific plot hooks, preperation, and a huge portion of the storyline dies with him. It is very difficult to invest a new character with the same sense of purpose in the existing story.
Is the second style of play more linear? Of course it is. But if people are enjoying it, that's okay. Also, if they complain that save-or-die or rust monsters are disruptive to their style of play to the point that they would get upset at having their expectations for their character arc dashed, it isn't because they have to grow a set of testicles. It is because they have a different viewpoint about what the game is for than you do.
Finally, there is the problem of DM's who like to use save-or-die, rust monsters, and other such mechanics because they are an easy way to use the rules to frustrate or bully their fellow players. That doesn't mean that save or die mechanics are to blame directly for that DM's behavior, but it has left a lot of bad feelings towards those mechanics by some players.