You can build the most secure app and install it on the most secure hardware, but if people simply instead use their private phone to chat with other people on

You can build the most secure app and install it on the most secure hardware, but if people simply instead use their private phone to chat with other people on their private phone using a random app from the App Store, then it just doesn't matter. You can then write as many rules as you like about how you're not allowed to that, but if you don't follow up and enforce those rules then people will keep doing it. Bottom line, no one cares about security enough to compromise convenience, and unless you start to literally throw people in jail they never will.