Or if you're primarily reachable by an app that can't launch until AFU, the phone reboots silently and you don't realize it, and you're incommunicado.

Some time later, you need to do something on the phone, you unlock it, the app starts up, and a flood of messages pours in. Wow, some of those would've been really useful to receive in a timely fashion! Whoops!

Locking the SIM is considered part of the feature on GrapheneOS AIUI

I don't remember the PIN to my work SIM card. Can't wait to lose my work phone halfway through a conference / business trip because I haven't touched it in a while.