The "your X has been replaced" pop-up doesn't handle the situation where an attacker knows your passcode.
I think you might also be failing to account for situations where you aren't in possession of your phone for an hour or two. Imagine if police in a foreign country take your phone for a couple of hours and then give it back to you. Or you leave your phone in a hotel room to charge for a few hours. Or your phone gets "misplaced" for an hour after going through the airport x-ray machine.
There are many targets other than journalists too, such as people in the USA who develop export controlled technologies, certain tech company employees, defense contractor employees, other government employees, etc. I don't think you can expect every potential target to constantly set their iphone to lockdown mode.