I feel like phones should just have a "scrub anything that isn't ASCII text" option for paranoid folks. No unicode, no emoji, no media. I mean, I guess they could still f*ck that up, and maybe it'd be admitting defeat, but still.
AIUI part of the problem is that iMessage is a poor legacy design and relies on generic macOS serialization primitives. It's not like HTML where you can just scrub all the tags out. This can't be changed without breaking compatibility due to end to end encryption (the server can't adapt between versions). So there is a big attack surface inherent to the design, and Apple are stuck with it.
Are they?
They have an enormous level of control over their deployed software. It seems like they could push out the required changes and nearly all clients would support it in no time.
For the remaining clients it seems consistent with the feature to simply block incoming messages in the wrong format.
Now, whether they'd want to implement a feature that essentially advertises that they have a security problem is another thing. (the answer is quite obviously no, regardless of what's best for their users)