Contrived.
For me, the best part of D&D is how the story writes itself. Week in and week out, I have no idea what we're going to script next. If I remove the threat of death, I'm not letting the story unfold. Some of the most memorable stories I've ever had came from character deaths, even group deaths. [snip]
Absolutely none of this would occur if I, the DM, had decided in advance to fudge rolls because I needed the PCs to survive, perhaps to fulfill a prophecy I wrote in advance or whatever railroad design you want to imagine. It is no different if the players were collaborating on this effort to fudge away anything lethal so you could make the story you intended to make.
That, to me, would be the ultimate insult to the game: to predetermine how it should turn out.