Actually, I can totally imagine this. Not just after Season 5, but later, maybe around his 40th birthday. Or maybe if he will be confronted with cancer again. Or if Gus begs him to do it because he learnt that smoking is bad (Children often do that, in my experience. My girlfriend smokes and her nephews and nieces pester her about it every time - I support that, of course ;o), because I never smoked). And if he really, really wants to stop, he will succeed because he always does. But he will be incredibly moody and grumpy when he tries.
He does care about health issues - at least sometimes - and it's not cool anymore to smoke these days and it's forbidden at most places anyway (at least here in Germany, and I suppose it's rather more strict in the US) so he probably will stop because potential sex partners may even reject him? Well, not sure about that. I mean, I hate smoke, but I wouldn't ;o)...
Then again, he might not quit smoking because he's afraid of gaining weight if he does.
You know what? I want to read fic about this. How about a mini-challenge or at least a prompt about that?