People’s phones got compromised by NSO sending images to them via whatsapp that used an exploit in one of the image libraries to run a malware payload. The security boundary isn’t about whether you can see your own messages, it’s whether bad people can root your phone by getting untrusted code to run. That’s a very different proposition if iMessage is a single codebase that they fully own end to end versus it has a plugin ecosystem. Having such a plugin system widens the security boundary by adding a much larger codebase that would require trust.