1) I think an inconsistent screen tint at viewing angles is an objective defect when its not exhibited with every screen of the exact same screen model, and there are screens made by a different manufacturer that do not exhibit the characteristic. If viewing angle color distortion and fading was a trade off, and acknowledged or offset as a way to solve another issue or boost some other performance factor, then one could argue that its a feature. I have heard no discussion that screen distortion is utilized as a way to offset or boost to another performance factor. Call it a defect, for a characteristic of an inferior manufactured screen. Either way, its objectively not as good of a screen as those not exhibiting the characteristics.
2) You don't need scientific tests, when the issue is easily and objectively seen with the naked eye. For people to come into a huge thread and say "what's the big deal" and call everyone nerds or OCD is A. ignoring half of the reason the forum exists, and B. gaslighting inconsistencies with the screen quality as peoples imagination.
3) How would taking pictures of screens be the worst possible way to show the characteristics as long as you are showing them all taken with the same light, camera, next to each other etc. etc. Taking a picture of the Apple store display stand, where you can plainly see differences with all of the phones at the same brightness, on the exact same web page, taken at the same time, with the same camera, is plain as day showing screen distortions.
It may or may not bother every iPhone user. It certainly bothered me, which is why I currently have a 17 pro that doesn't have the issue at all. I haven't checked to see what screen it is, because I don't care, as it looks fin to me unlike my 17 pro max and Air which faded and greed out at the slightest angle.