Like many others here, I'm an electronics technician and have been in my trade/profession for 50 years. Every technician who has ever done bench work has seen the contact degradation problems caused by arcing hundreds of times over and the lightning connector is indeed a problem. I have had the same issue as many others, wasted an inordinate amount of time making contact with the so-called "Genius" bar and turned up to my appointment only to be cheerfully given a new cable by an Apple employee who had no knowledge of electrical arcing and didn't want to listen to my explanation of how I had not contributed in any way to this problem.
IMHO there are several contributors to the problem and the probable poor design of the aptly named "Lightning™" interface. Apple employees who have put it down to moisture are partly right but it does play a part in all electrical arcing. What I guess they are overlooking is that water vapour in the air isn't ever going away and will always be absorbed by minute contaminants, mainly dust and grease particles which form a bridge allowing arcing which is just like "lightning" conduction and which can happen at pretty much any energy level.
Several others have pointed out that the blackening of the connector contacts is burning, not corrosion and they are right. How this burning happens can be simply put down to the fact that most of us forget to switch off our chargers and wait for the energy to dissipate before unplugging, and a tiny arc "spark-erodes" the power contacts when we unplug. Over a very short period time the very small amount of gold flashing on the contacts is penetrated to and the copper contact is able to oxidise and form a carbon film. In a perfect design, the contacts would be solid gold, there would be adequate transient suppression on the power conductors, more contact pressure and so on, but the design is simply not perfect and the reality is that Apple does not see it as a sufficiently serious issue to warrant any sort of a recall, which would be a logistical nightmare anyway, nigh impossible given the volumes of iPhones sold.
It's just disappointing that somebody from Apple is not honest enough to admit that the design is lacking and promise that the next model will be better. I think we would all would prefer that to employees blaming moisture exposure and avoiding the real issue. I have great admiration for Apple but a simple act of honesty goes a long way to retaining customers for any business.
The best solution to at least lessening the problem is to always keep the contacts clean, use a little bit of electrical contact cleaner or even isopropyl alcohol on both the plug and the socket and always switch off and wait a minute before unplugging or plugging in the charger. The little dental cleaning brushes that you can buy for cleaning between teeth are great for cleaning the socket, and you'll amazed at the amount of gunk that accumulates in there.
