[Traubenfuchs]

> my understanding is users can't replace iMessage with another messaging app.

To be precise there is one "Messaging" app, that automagically uses iMessage (blue bubbles) instead of SMS (green bubbles) whenever possible. One can turn off iMessage in the settings, which will probably lead to your iPhone rejecting iMessages, making other iPhones only send SMS to you and also make your iPhone only send SMS. Whether that toggle prevents receiving and processing of malicious, invisible iMessages is an entirely different question.