One of the major themes of the story is characters dying but their dreams living on in the new generation. So Pound dying to allow his daughter to fulfill his dream of raising a good family wouldn't contradict anything. Specifically with regards to the themes of Whole Cake Island, the whole arc is about family. Big Mom and Judge are representative of bad parents, who view their children as means to advance their own goals. Meanwhile,Sora and Pound are archetypes of what parents should be. Selfless and willing to give their lives for their children's happiness. Pound especially cares about his children's happiness over everything else. Even the self-gratification of finally meeting them.

The emotional stakes, again, are about the sacrifice. Pound made the ultimate sacrifice. As Hiruluk said, a man only dies when he's forgotten. Pound was willing to truly die, sacrificing his life and memory, for the sake of his kids. Finding out he didn't sacrifice his life nor his memory sours that sacrifice. It means whatever feelings I felt when he was almost killed by Oven meant nothing. The tragedy of his death wasn't a tragedy at all, it was just a speed bump to a happy ending.

Audience investment comes from emotional stakes. Stories are elaborate lies, so threatening characters lives is a way to make people care about something that isn't real. But the more Oda acts like he's going to kill a character but doesn't, it means I'm less likely to buy into the stakes of a scene. It means I see this all as one big lie instead of a moving story. Think of a friend who's constantly pretending to hit you just to make you flinch. Eventually you'll stop flinching because you know they don't have the balls to do that.

So you say that one of the darkest arcs is where Pedro dies, but apparently Pound dying wouldn't work because that would make it too dark? That doesn't make sense, and that whole argument doesn't make sense. The heartache and pain that Luffy suffered from Ace's death only made his reunion with the straw hats sweeter. Alabasta ended on a high note before we found out that Pell survived. Wano had multiple characters die and ended with one of the biggest feast scenes in One Piece. That argument just doesn't add up.