Can’t law enforcement already read those messages by getting a warrant to seize the suspect’s phone and attempting to break into it? Why do they suddenly need to preemptively break into everyone’s phone?

I’m going to give you the benefit of the doubt, I’m assuming you don’t know that likening anyone who doesn’t want to have all of their personal information viewed to terrorists and paedophiles is the classic “what do you have to hide?” authoritarian argument to spy on everyone all the time.

1. There have already been plenty of cases of data collected without a warrant just because they could.
2. Do you still want that data to be collected and used to prosecute you if whichever political party you don’t like get in and make something you like doing illegal?
3. It is impossible to make a backdoor that only goodies can use. The actual terrorists and paedophiles will use a non-backdoored system, meanwhile every criminal organisation and rival nation state will eventually find out how to use the backdoor and get everyone’s information.