Who wants to bet that Apple planned on a later release to screw up everyone's 12 or 24 month cycle and you end up paying two more payments this year then two more payments next year when they get back to mid September launch? $$$
Could this be a consideration? No doubt, though Apple's biggest customers, the phone companies, are probably more interested in this than Apple is.
The thing is, not everyone buys the new model on Day 1. Those who wait for Day 1 in this case are delaying their purchase by a month-and-a-half, which is not a good thing. Will they buy their next iPhone in 10.5 or 22.5 months, instead of 12 or 24? It's not a sure bet - if they're on a payment plan, they have a financial incentive to wait.
In my case... I buy in the early part of the year, not Day 1, so I'm not delaying or accelerating my purchases. By February or March, when I replace my iPhone 6, I'm fairly certain I'll have no problem getting an X without a wait (I'd be more certain if it wasn't for the AirPods precedent).