In many industries, once someone has physical access to a device, all bets are off. And when used correctly, TPMs can provide tons of value even when not encrypting the bus.
I am using TMP for this on x86 machines that I want to boot headless. If I need to replace the disk I can just do a regular wipe and feel pretty comfortable.
I'd use a Yubikey or other security token with the Pi, but the device needs to boot without user intervention and the decryption code I'm aware of forces user presence whether or not the Yubikey requires that.