Which position are you bringing it back to? Do you think a bare hand in a boxing position is going to stop someone hitting your head?
The wing chun guard is naturally more extended than a boxing guard because we don't wear boxing gloves. So, it's comparable to grappling styles, esp old Cumberland/Lancashire styles which evolved into catch, and old bare knuckle boxing which allowed throws and grabs to some extent anyway. The whole point about the sensitivity is allied to that too. Plus, the ninety degree (slightly + ?) angle is inherently strong (search for Andrew S's post linking to a study) so there's a reason we have that position.
Of course you don't want to leave the arm out doing nothing!
Dunno. When I was boxing I was taught to withdraw the hand as fast as it went out, but that soon became a reflex so whether you can say it was 'consciously' is a moot point.
Also karateka are taught to focus on bringing the hand back and some of them punch hard enough.
But agreed, the flex after your punch where the split-second tension leaves your arm and crooks your elbow ready for the next short punch can be trained as a reflex from the rebound.
Following on from what Terrence said too, I think the retaining contact is important, and Drew's point about range is spot on too. Many chunners fire off punches from too far: dan chi sao is practised out of range, chi sao gets a little exuberant and too fast... to me the really effective chun range is with the opponent between your man sao and tan sao range. Of course, what this may vary!
This is why with any punch, with any art, follow up footwork is all important. Over extension and leaving your arms out is a symptom of moving without the rest of your body, which partly comes from over-reliance on lazy chi sao (you know, we all do it, where you hit someone, they step back and you let them go instead of following up... or you follow up with arm-only chain punches instead of linking a la CK and following up putting your whole body into the next punch from a more assured position) and not practising on enough live, dodging, moving targets.
Just a few ideas.