You do understand the reasons right? iCloud, iMessages etc are an interplay of software in your device, software in cloud and some networking. They have to work together. A jailbroken phone can have a tinkered iMessage app. Supporting tinkered / unsupported versions is more overhead / possible security risk. Why would you expect them to take on that?
I agree with your point, though I can easily modify ~/Library/Messages/Attachments or ~/Library/Containers/com.apple.MobileSMS/Data on macOS, which is a platform similarly secure to an iPhone when you take sandboxing, sip, etc. into account. So Apple already supports macOS, which can be tinkered, no? I guess the argument is that by locking down iOS, a platform which was never open, they just never needed to handle that class of security risks which are introduced when customers have more freedom.
I did actually discover a bug in iOS (related to photos which have failed to sync to icloud for the past four years) by investigating why there were over 23k empty folders in ~/Library/Containers/com.apple.photos.ImageConversionService/Data/tmp/, fwiw.
You're right, but I could see Apple acting on iOS alone, just because of scale.
There are some orders of magnitude more iOS/iPadOS devices than macOS devices.
There's a bunch of ways we could debate ourselves about things Apple would or wouldn't do, but at the end of the day, Apple is going to do whatever is in it's best interest.
My computer is jailbroken, I have sip disabled yet icloud and imessage work. So what makes my computer different from my phone?
Who bears the security risk? The user.
The only security risk for apple is a job security risk if the ecosystem lock-in is eroded.
The risk to Apple is everyone else’s trust being eroded by having their ecosystem compromised. Why risk it?
It does not make sense for someone to lose faith in apple because someone else was able to run a modified imessage app.
For apple to entertain this idea is to create more problems for themselves in the long run.
If you understand this from my perspective, which is that these actions are motivated by a desire for more control. It begins to make more sense. If you can benefit from increased control, then these unreasonable expectations of security against your customers (held by other customers) are a desirable problem to have as opposed to an increasing burden.
Imagine the same being said of a car manufacturer. "We need to install security measures in place to prevent you from modifiying your car because other people may see that modified versions of our cars exist and that tarnishes our brand somehow".
As long there is no reason to allow companies to exert this control over consumers, it just allows the manufacturer to enforce a total monopoly on the post-market.