Listen guys, I'm making these three claims.
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There is a world of difference between being a tough DM and being a killer DM. By it's very nature, the position of DM has the power to do a TPK every round of game time. Good DMs don't use it- they let the parties do it to themselves.
The various "resets" in D&D exist partly because they show up in some of the source material that inspired the game's creation, but also as a bit of a safety net so that PCs built up over time have a safety net.
In written fiction, this is often replaced by "plot protection."