FWIW, Bitlocker already can store recovery keys in AD. It has been a feature for a long time.
Keeping the key stored on the machine, TPM or no, is also less secure than keeping it somewhere else. If someone steals your laptop, you deny all access to the key on the server and they can't get it even if they could guess the pin (or the user wrote that on the bottom of the computer), and there is no way to use an offline method to extract the key from the TPM because it isn't there.
Computers have these neat things called "local filesystems". They're a real hoot. Maybe you can get one on your computer.
This seems like it isn't worth the cost in authoritarianism?
For that matter you could still implement even that with just a secure enclave that will only release the key given the correct PIN (and then rate limits attempts etc.), but then does actually release the key in that case and doesn't do any kind of remote attestation or signing.
So...a TPM?
> This seems like it isn't worth the cost in authoritarianism?
You know what's really authoritarian? Having your computer practically only decryptable by some remote directory server, potentially not even under your control.
Unclear why this requires a TPM. Boot the system from a static unencrypted partition containing no sensitive data, display the login screen, when the user authenticates the system uses their credentials to get the FDE decryption key from the directory server. Bonus: Now the FDE keys are stored in the directory server and if the system board fails in the laptop you can remove the drive and recover the data.
An attacker with physical access could modify the unencrypted partition to compromise the user's password the next time the user logs in, but they could do the same thing with a hardware keylogger.
> And for casual home Windows users, it lets them log in with a 4-digit PIN or with biometrics, so it's got usability benefits for them too.
This could be implemented the same way using Microsoft's servers, given that they seem to insist you create a Microsoft account these days anyway.
It's not clear that unsophisticated users actually benefit from default-FDE though. They're more likely to lose their data to it than have it protect them from theft, and losing your family photos is generally more of a harm than some third party getting access to your family photos.