Display variance has always been an issue with iPhone suppliers/manufacturing. It isn't glue drying. It isn't only the first batch, though I would imagine that, if Apple is using multiple suppliers, that once the demand slows down, they can use the better supplier more exclusively.
Attached is a photo of the two iPhone 6 in my house. Both were purchased about 9 months after launch. Both are the same exact model purchased at the same store on the same day. Both are over a year old and have been heavily used. Photo was taken on a 5D mark iv DSLR with white balance calibrated to 6500k using a neutral gray card.
There is clearly a difference between the two. As a photographer, the phone on the left is far more accurate color-wise. In fact, it is nearly a perfect match to my calibrated studio monitors. The phone on the right is too cool AND heavily shifted toward magenta (which will give the phone on the left the appearance that it is shifted toward green).
The annoying thing about taking this side by side just now is that the more correct phone on the left (my phone) is noticeably softer than the right, where text is tack sharp. So there is yet another difference in the panels.
I was one of the people that had a scuffed iPhone 5 out of the box many years ago. Apple let me keep the original when they sent a replacement. When the replacement arrived the display was far more cool than the scuffed model. But beyond that, the replacement had a significantly worse color gamut. Colors were washed out and highlights blew out to pure white when the other phone still showed detail in those areas. I kept the scuffed phone as the replacement was inferior in every way.
So, not to complicate things, but be on the lookout for differences in color temperature, sharpness, and color gamut differences.
As there get to be fewer and fewer features that can be added to distinguish new models, I always hoped Apple would push toward greater consistency. You can't tout these huge advancements in display quality if the displays are so varied. I remember them touting the cameras used to cut the glass parts of the iPhone 5 to ensure a perfect fit. I think it's time they put that attention to detail into factory calibration of every screen that goes out the door. Relatively cheap ASUS ProArt monitors come with a factory calibration report in the box.
With no way to calibrate iOS displays ourselves, a factory calibration would be better than nothing and prevent a lot of returns from people playing a panel lottery.
But most people simply do not notice or care. For instance, it absolutely blows my mind that high end TVs are not factory calibrated. TVs have one job... accurately displaying content. Yet there isn't a single TV on the market that comes close to accurate on any preset out of the box. Factory calibration can't replace true and regular calibration in your environment, but it's so much better than what you get out of the box.
