Yes, [the thought of] getting locked out of my Apple ID is the biggie, since I migrated everything mission critical to that email address over the years and later put my important documents in the iCloud Drive tied to that ID. [and put almost all of my passwords in the keychain for that ID.]
While I would feel violated about having my phone stolen, I could get a new phone immediately. Undoing the damage of being locked out of my ID would be painful. I’m going to spend the weekend either protecting that ID or making it less potent.
And if you use iCloud Photos, they could delete everything or lock you out of everything. You definitely need a backup, that is not tied into it. And if they have your phone and reset your AppleID password, couldn't they then lock and remote wipe your Mac(s) and other devices? Documents, photos, everything.
Even with a full copy on a different Mac, you'd be screwed if it was signed into your iCloud account.
Would you be locked out of your Time Machine backups? (Not sure).
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