I think the shift was already in place by the early-mid 80s. Also a lot of people picking up the game as kids (myself included) never really did that anyway.
I didn't really undersatnd that style until I played in a game with the parents of one of my friends that had been running since the 70s, and it was eye-opening in a lot of ways.
But either way, experiencing that style showed me how it could really work, and how death would be perceived in that style. And there's plenty of historical evidence that that's exactly how Gary ran his games, and likely Greenwood as well as others - Undermountain/Waterdeep follow exactly the pattern of "a town built on a megadungeon, oh, and look at all the retired PCs that are now NPCs".
In that style, character death just... isn't a big deal. Once you presume a party stays together, having characters die creates a lot of problems with continuity, power, bringing characters in, etc.
If there's no "party", and it's just "whoever shows up", then a character being gone isn't a big deal, since there's no expectation. If there is a party and plot, then you've suddenly got a missing role and their narrative threads end.
If your character at level n dies, it's again not a big deal. You can either bring in one of your characters near that level, or the group can just play their lower level characters for some sessions. Since churn is a thing, they won't be permanently behind the rest of the party. In the "one party" game, you have to manage that - do they come in at level - 1 and be behind? Do they come in at level, making it feel like the death didn't matter? Do they (probably not) start at level 1?
Since there's no party, "why do we let in this rando and where do we meet them?" is unimportant. The mechanisms for "put together the party for the week" already exist, so the fact that an additional new character joins the pool (if they even do) doesn't matter. In the one party game, where they're wandering about, how do they meet them? Where do they meet them? Why do they decide to travel together?
A lot of problems with character death just don't exist in the open-table style of play, and become much bigger issues if you assume narrative and a stable party.
Also, to be clear, I'm not saying "everyone played like this". I am saying that that's how Gary and a lot of the other early influencers played, and that the game evolved around that style. And as the common style changed to consistent parties, some of the rules that made sense didn't work and needed to change.