Perfect example of negative spin. One commenter was surprised at quickness of the slips "at those prices" and you think the slips are not fast enough "at those prices". It's all about mindset. You think the prices are too high so you want to confirm that by building a case that pre-order delivery dates aren't slipping fast enough. Other do the same in reverse. They also think the prices are too high so they invent another theory to explain why pre-order delivery dates are slipping, namely the tired old "Apple is intentionally limiting supplies to make it look like demand is high."
My own take is that it is a combination of the price and the relative lack of must-have features that weren't highlighted that convinced people that the phones that they have is still good enough and don't need to upgrade right on day one. Just based on this forum and the pre-order delivery dates not falling fast enough makes me think that a majority of people who are upgrading at this time are the die hard Apple enthusiasts and maybe a handful of people upgrading from iPhone 6 or earlier phones.
No hard facts, just my own thoughts, take it for what you will.