iCloud Backup being insecure does NOT mean iMessage is "backdoored": if you don't use iCloud Backup--and there is no reason to do so, as Apple (notably unlike anything Android) has a
really good local way to do highly-secure backups using iTunes--my (rather firm) understanding is that, even if you turn on the iCloud iMessage sync (which is also optional: I do not have it on, for example), iMessage actually is pretty damned secure (with no key escrow).
I appreciate that a lot of people--maybe most or even "almost all" people!--use iCloud Backup, but semantics matter as you present it as if iMessage is insecure, when not only the "real" but the only issue is iCloud Backups (which we should be making sure people don't use, with targeted education campaigns, as it isn't like using Signal could ever solve this issue for all of the other personal data the user had).
If you want to make this kind of argument, you need to be doing it from a place of being more informed; like, you could be trying to poke at how the protocol had a weakness--found by pod2g's team--with respect to Apple's ability to inject new keys for existing chat participants, an issue I am not sure they fixed (or even could fix, as it is arguably a UI security problem... but maybe they did, as I don't always follow the blow-by-blow); but this indirect argument you make is FUD.