If Apple knew EVERYTHING, then why not start producing it right NOW?
There are always unknowns up until the last minute for any hardware/software release.
Prototypes are created over and over again... then things are tested in the real world that like cellular strength... then tweaks are made and another prototype is created.... and so on... Just because something is on paper doesn't mean it can be produced in the real world exactly like it was on paper.
Most of the people commenting on this website or any other forum for that matter don't have any idea how manufacturing/Supply Chain works.
I Understand because most of the manufacturing is done outside US now so lot of people don't work in manufacturing in US anymore.
I am sure Apple will still be fixing manufacturing/quality issues till the last minute, nobody is 100% sure which technology works till they start making in large quantities, they evaluate the risk & decide to go with it.
If they don't take the risk then some Chinese phone manufacturing company will get hold of the technology & bring it to market before Apple does.