@King:
Well, he still had potential, but instead of becoming stronger like Luffy after Ace's death, he became lazy, fat and weaker.
Heck, that's pretty much the story of every Straw Hat + Law. Friend(s) die(s)/gives up leg, must become stronger to realize dream/will.
Luffy's mind snapped at Ace's death, and he broke down to an incredible degree when he couldn't protect his nakama from being sent away by Kuma on Saboady.
Had it not been for Jinbei's words when Luffy was having his breakdown after Marineford ("I'm weak!") who knows if Luffy would have come through the other side of it.
Moria still having the ambition to become PK (albeit with an army of disposable zombies doing the work for him) is pretty incredible, he certainly doesn't give up easily.
–- Update From New Post Merge ---
@King:
Well, Robin, Brook and Law pretty much lost everybody they knew in one swoop.
Aside from losing a bit of sanity in Brook's case, they're all still following their dreams and such.
Robin and Brook I can agree with to an extent.
Law lost everything he knew and degraded to the point of joining a murderous pirate crew just so he could kill and destroy as much as possible in the time he had left.
That one I'd have to disagree with.