Poettering mentions three: The obvious 'basic' one, and two advanced scenarios focused on a thief stealing the computer and then returning it. I recall exactly one case close to the these latter two scenarios: When Mossad stole a Syrian laptop allegedly containing nuclear weapon program information and returned it[0].
However, I do not think Poettering's proposal is going to even annoy Mossad-level operations. The same people who can steal your harddrive and put it back in without notice, can for example replace your mouse with a BadUSB device and get all the info that way. Once they got hardware access, and were able to return it without the owner noticing, they have multiple ways of getting what they want. On Linux we can't be assured of controlling the hardware Apple-style.
So this is a proposal which probably doesn't stop the advanced and rare scenarios it thinks it stops. In return, we get more complexity by splitting the system into at least three components*, and going back into the partitioning mess ("You only gave 300GB to /home, in order to download the ISO you want you need to repartition the system").
IMHO, the cost-benefit ratio for splitting the system isn't good enough. We should try a simpler solution with the main purpose of stopping the basic scenario. Maybe dm-crypt everything, despite the minor speed loss? Or use fs-level authentication/encryption a-la fscrypt? Either solution still allows people who want to split the system to do so.
[0] https://www.theregister.com/2009/11/06/mossad_syria_trojan_h...
* The authenticated base system, the encrypted base system, and at least one encrypted home directory. In most system there will also be an encrypted swap partition.