It's not unreasonable for Apple to appreciate an aesthetic that is less accessible considering they provide the tools for users to customize the device for their needs.

The majority of people have no problem with the contrast of the Messages app, or at least it's never been a complaint that I've heard. That's not to say we shouldn't consider those who do have problems, but, again, there are options for those people.

That is in fact one of the accessibility settings. Increases contrast front the blue bubbles too, and color contrast between foreground and background colors system wide.

Good, the defaults are so insidiously-designed that you'd be forgiven for assuming it was naturally user-hostile.

This might sound strange but most people with iPhones actually like how their phones look and behave. Personally I’m not the biggest fan of this era of UI design, but it’s silly to assume hostility on Apple’s part. Every year they announce even more accessibility features, and it makes sense to because they really want more people to actually be able to use the phones they sell.