You are correct. The IUP allows a customer to upgrade to a new iPhone, once 12 payments have been made. If Apple releases a new phone in September 2018, and the customer enrolled in the IUP for the iPhone X in November 2017, then that customer would be required to make an additional 2 months of payments, prior to being allowed to upgrade to the new phone during a September 2018 launch. The customer will essentially be "resetting" their upgrade cycle to be back in line with the traditional iPhone release schedule. Alternatively, the customer can keep the iPhone X for an additional 2 months, and then upgrade via the IUP, thus preserving the "November" release schedule of the iPhone X.
It is important to note, that once a customer has chosen to upgrade via the IUP, they are not forced to return their existing phone. They can keep the phone (and would then have 2 IUP loans), and can either continue to pay off the balance via monthly installments, or make one lump sum payment to fulfill the loan. Some customers choose to keep their phone, and resell on their own for a profit, to help recoup some of the funds.
As far as it being wasteful, it really is a matter of personal choice. Apple has been fairly clear in their IUP documentation as to exactly how the process works. While I can understand that the "marketing" hype creates an impression of yearly launch day upgrades, in reality that is simply not how it works.