Be careful what you wish for. I think that we Red Sox fans might be better off with Andy Pettitte hanging on until the bitter end, with that bitter end coming midseason and forcing the Yankees to scramble for a replacement on the fly. If Pettitte certainly retires in the offseason, then his rotation slot will certainly be filled, either internally or via trade, by a pitcher who will be more effective than a "falling off the cliff" Andy Pettitte. The Yankees have plenty of time to hunt for at least a middle-tier starting pitcher at a reasonably low trade cost right now, whereas a midseason trade for a clear upgrade might cost them more in prospects. I think that Pettitte's retirement would force the Yankees into making a deal now, because the alternative is, as you say, a rotation of Sabathia-Burnett-Hughes-Nova (or equivalent), and with the obvious Bubba Crosby 5th starter that the Yankees will be "comfortable with going into the season."
I agree with your general point of if Pettitte retires they most likely go out there and get someone better. If he retires*, they'd have to do something outside the organization to get above average innings for this very expensive club of theirs. And it seems pretty reasonable that they could stand pat so long as Pettitte returns. Although, I'd want to hedge that bet of inactivity because the rotation could still be upgraded fairly easily and painlessly considering their system and the Greinke deal. The trade options are limited but there still are some that would make sense.
I don't think Pettitte is part-cyborg, but as far as soon to be 39 year olds go, I think Pettitte is a good bet to not fall off a cliff next season. He isn't dependent on a dominant FB to be a good pitcher (just going by pitch value since 2007 his FB: -12.8, -4.9, -6.1, -1.4) his velocity has been steady for years now (fangraphs is showing a drop in his 2010 cutter velocity but without seeing him all that much I'm not confident in that # considering FG pitch recognition issues) and the guy just knows how to pitch and keep the ball of the center of the plate and away from the barrel of bats (since returning to the Bronx his LD% are below his career avg while his HR/9 hasn't really been an issue). Overall, I'm with you, I'd like Andy to just take one step closer to becoming more like Favre and come back next year, I'm just not expecting a significant drop-off from him though. On the surface his profile seems to me more like a gradual decliner.
* still have my doubts he actually retires since he does this every year, but yeah probably not to this degree.