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.
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My computer is jailbroken, I have sip disabled yet icloud and imessage work. So what makes my computer different from my phone?
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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.
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