That's premised on the attacker never having write access to the encrypted partition, which is the thing storing the FDE key on a remote system or removable media does better than a TPM. If the key is in a TPM and they can extract it using a TPM vulnerability or specialized equipment. Or boot up the system and unlock the partition by running the original signed boot chain, giving the attacker the opportunity to compromise the now-running OS using DMA attacks, cold-boot attacks, etc. Or they can stick it in a drawer without network access to receive updates until someone publishes a relevant vulnerability in the version of the OS that was on it when it was stolen.

Notice that if they can modify/replace the device without you noticing then they can leave you one that displays the same unlock screen as the original but sends any credentials you enter to the attacker. Once they've had physical access to the device you can't trust it. The main advantage of FDE is that they can't read what was on a powered off device they blatantly steal, and then the last thing you want is for the FDE key to be somewhere on the device that they could potentially extract instead of on a remote system or removable media that they don't have access to.