To unspin your statement, Beeper is providing Apple users with more functionality by way of giving them the ability to send iMessages to their Android-user friends. This is beneficial for Apple users, but Apple has a financial incentive to restrict their users so that the only way for other people to interact with them is to purchase an Apple device, so they are harming their own Apple device users by trying to cut off this functionality.
I disagree. Much of the value of iMessage is the high degree of confidence in its security. Apple controls, end-to-end, the delivery of iMessages from device-to-device. This includes how it handles storage of messages, attachments, app diagnostics, crash reports, etc.. Beeper is an unauthorized user of this supposed-to-be closed-system, and I think it's deteriorating the trust users have in the platform entirely.
How does accessibility hurt security? Security issues for a messaging platform manifest when messages can be intercepted. Having the option to send or receive from more users does not put a man in the middle.
If you are doing something that definitely requires E2EE that you are certain works and forces the user on the other end to comply, then maybe I could see your point. You would be in the vast minority of users and it's debatable that your use case is worth gutting the functionality for the majority of users who would benefit from being able to iMessage their Android friends, but you would still have a valid use case. I would tell you in that case to use an open source E2EE solution instead of Apple's closed source one though, given that privacy is such a concern.
There are more threat vectors than man-in-the-middle attacks. Beeper Mini does not make the iMessage protocol any less safe. It's end-to-end encrypted. Great. No problem there.
The problem is: the client application itself is a big unknown. Beeper says it's secure, and it very well could be. But we have no idea. But if it receives, decrypts, then sends sensitive information, as diagnostics, back to a a central server unencrypted, for example, that's obviously a huge problem.
In that case Beeper Mini themselves are the man in the middle... But that aside, yeah, that's a problem with all closed source E2EE services. If you are a journalist in a warring nation or are being tracked by state actors I would hope you are not using iMessage to stay safe to begin with.
