Actually lockdown is better. It leaves E2E encryption alone, but restricts attachment types, which should be enough to block the initial exploit in the chain.

Disabling iMessage would fall back to SMS, allowing messages to be snooped / modified in transit.

Hopefully they’ll also have a way to disable RCS, since it allows attackers to modify messages, and also has a larger implementation attack surface than SMS.

No, Lockdown Mode doesn't disable iMessage.

"Most message attachments are blocked and some features are unavailable."

iMessage with blue bubbles still works in Lockdown Mode. I think GIFs don't display properly and certain other attachments, but I can share photos, audio clips and video so I otherwise don't really notice that Lockdown Mode is enabled.