@Greg:
Your enjoyment (and analytical prowess) of One Piece will be increased considerably if you can let go of thinking about Oda's refusal to kill anyone unless the plot DEMANDS it.
The question ceases to be, "Why isn't that individual dead?", and becomes, "That's interesting, how can that individual be alive and what does that mean for the story?"
Instead of thinking that it negates someone's sacrifice (which btw, I'm in agreement that it does in certain cases) it can actually serve to bolster the significance and impact of actual death. But yeah, having suffered years with that frustration, lemme say from experience, letting go of it makes everything much more enjoyable.
I should note with all that said, I have mostly let go of it except for when it crops up in a particularly annoying way, it no longer surprises or annoys me too much lol. I've been following the series since 2013, so I am used to it. And I have heard the point many times (and I agree with it in several ways) about how rather than focusing on why, logically/structurally/thematically/etc someone should be dead, focus on the opportunities Oda keeps alive (heh) in the narrative by keeping said character around. The only trouble is that, with the occasional exception, I don't think Oda really takes advantage of that. One of the few times I thought it worked would be Galley-La surviving the Buster Call (as well as the absurdly over-dramatic one-panel moment where Paulie got stabbed in the initial assault of Enies Lobby lol) as I absolutely would not have thought them dying there would serve anything, and am very glad they're still around lol. Same with Hatchi to an extent. While him dying then and there would've been powerful, I also like him having a cute life with Keimi.
Thing is, especially with the most infamous/egregious examples, I don't think that argument holds much water. For example with Pell, I see really nothing gained in his survival (he's just appeared in non-dialogue cameos involving the Nefertari family, Igaram and Chaka already have his "role" if you can call it that covered pretty well, and I can't imagine him doing something significant in the future, beyond being among a mass supporting Luffy near the end of the series) and in exchange you loose the weight of his sacrifice entirely, making the "threat" of the bomb nonexistant, (honestly there's another trope Oda relies upon, the cataclysmic finale device which brings the disparate elements/characters of a standard arc together for the climax - Crocodile's "bomb," Enels' "death game," Spandam's "Buster Call," Ceaser's poison gas, Doffy's "birdcage," etc, etc - and usually finds a way to make all or most of the nameless mooks or really-side characters explicitly survive, thus discrediting said device past the tension-filled moments of their use) giving Vivi a bit more baggage after the War & letting her (and the audience) connect a bit more with the losses of the conflict. It's clear the war was short but devastating, and yet with no familiar faces lost it removes us quite a bit from any sense of negative consequence or pain from the war. Instead it's just happy times ahead. Again I don't mind that, I love One Piece's positive attitude, but just arbitrarily and cheaply cutting away potential consequences from all scenarios feels unrewarding. Obviously I think One Piece is far better than not (it's my favorite manga/comic of all time by far, one of if not my favorite stories as well) but this is just something that bugs me, though increasingly less so.
As an aside, Igaram also got this treatment in roughly the same part of the story (the double-whammy was what seemed to really hammer in the death-fake out trope in Oda's writing/the fanbases expectations) but while that too is slightly annoying I'd say it works. Oda capitalized on that moment by making Igaram a really fun and endearing character (Pell, while noble and cool, didn't have much depth) and provided a great gag/plot-device later with his impersonation of Vivi. Plus it also reinforces Robin's disloyalty to Baroque Works, and invites the question of whether she spared him or did a purposefully sloppy job.
But the vast majority of these survivals such as Pagaya, Bellamy, (Who I was open to being worth it, but ultimately imo his part in Dressrosa didn't warrant it) Gekko Moriah, (though with Kaido's upcoming arc, I'm anticipating this being changed) Bon Clay, (I can understand him coming back to help Luffy near the end way more prominantly then someone like Pell would, but I'm still conflicted about that - his sacrifice was just so moving and I cared about him so much more than Pell, even though I was pretty sure he'd live because of One Piece's conditioning, and I really really wish Oda had let him keep his sacrifice) just don't work for me. I am mostly over them like I said, but as this rant proves I can definitely get really riled up by it from time to time lol.
And it's all the little things too, from Oda including random panels just to assure us random masses of people survived x or y disaster, from all these characters talking about "killing" or "ending" each other or being afraid of others for their lives even though the history of the narrative makes all this dialogue seem rather empty when you think about it, (Law's talk of "pulling the trigger") it all just adds up lol. It lessens individual character's threats/seriousness, (when supposed psychopaths or thorough-killers only half-kill people out of choice, laziness, or no good reason) lessens the severity of mass destruction or battles in general, and is just a tension-killer. I mean, we all know Luffy & co. will not die until the end of series, if at all, (I mean, Oda "might" pull a twist or something, but considering One Piece's spirit/style I find that highly unlikely) but that doesn't mean these other elements don't make the dangers the Straw-Hats go through feel any less serious. That said, Oda hasn't been shy in some cases with killing off incredibly unimportant characters (Higuma the Bandit, - not technically a flashback death as it's structurally the first part of the series - Mr. 11, Roshio "the executioner, etc) and increasingly in the New World, nameless mooks.
I try not to think about this stuff most of the time & am able to really enjoy One Piece the vast majority of the time, but it does just crop up every so often for me. If Oda reveals that Oars Jr., Vergo, and/or Monet are still alive (still thinking he probably will lol, and probably won't have too many of the old WB pirate commanders/allies actually be offed by Weevil) I swear lol. Try not to let the rant intimidate you, it really doesn't bug me most of the time, but it really can irritate me lol.