I'm running the third campaign in my setting. I haven't really used any explicitly world-ending threats so far, no TPK at any point heretofore would have forced me to stop running games there.Lots of campaign worlds are independent of any given PC or group of PCs. There are things happening, locations present and factions moving all the time. A "campaign" is when a group of players creates characters that interact with that stuff. When the "campaign" ends (successfully or otherwise) the world continues on.
Or, more modern: just because there is a TPK for one party, Elturel is STILL in Avernus and someone has to save it (or not).
And "Elturel is still in Avernus" is the sort of thing that I don't think will matter as far as one PC death being easier or harder than a TPK, for various reasons.