I haven't noticed a supply and demand issue with the 6 in the US. There is with the 6+. They're hard to come by. I don't see how Apple could have underestimated their sales that much. Yes, there were production woes that nearly put off the 6+ release until October or later, but still...they were way short of supply. I hope they get caught up by mid December.
Sent from my ancient but trustworthy iPhone 5.