It would be great to think that Apple could have enough units ready for launch or could delay their launch date to accommodate, but the reality is they have to contend with many factors in making those decisions.
First of all, September is the end of their fiscal quarter. In order to deliver on their projections and attempt to minimize their YOY losses they have to continue to release their iPhones by the end of September.
October begins another fiscal quarter which relies generally on Macs, iPads and holiday sales for their iPhone. Shifting the iPhone would take away from their July quarter and their October quarter as well. Being a publicly traded company that has shareholders and investors, they unfortunately have to ensure that they deliver on their EPS estimates. Apple's business roadmap is set out well in advance of knowing whether or not their manufacturers can deliver solid yields or not.
It's unfortunate that these types of business implications and strategies have to impact the customer, but at the end of the day we are really complaining about whether or not we got a brand new device on launch day or whether we have to wait a bit. I still have a couple weeks to wait for you iPhone 7 Plus to arrive and I'm okay with that. I spend my time checking out cases and reading reviews and amplifying my excitement.
With that said, though, I get that there's got to be frustration of being told you're getting it on launch only for it to not happen. That does suck. But hopefully it won't be too much longer!