>terminus decree
>only use this when there's no hope for the imperium
>Tyrannids sweep through segmentum solar
>make landfall in Holy Terra
>Imperial Palace is on the verge of falling
>suddenly, at the last minute, the entire Grey Knight chapter shows up with Kaldor Drago at its head!
>they immediately rush into action and...
>...break into the throne room to make sure the Emperor is still sitting there
>leave

it makes sense to have a contingency to prevent the Emperor from going rogue or insane by trying to leave the Golden Throne and collapse the Imperium
but the very specific directions of the terminus decree just doesnt mesh with the idea that it is to be used in the 'darkest hour' of the Imperium, there are so many potential threats and potential scenarios for the 'darkest hour' where checking to make sure Big E is on the throne is a waste of time, and other scenarios where Big E getting off the throne is actually a good idea

it just seems too narrow and specific
and thats WITHOUT getting into the fact that between their extremely limited numbers and extremely limited political power (cuz of muh secrecy) they Grey Knights would stand no chance of even being in a position to enforce their decree, without getting into how the fuck they'd put a resurrected/reincarnated/ascended Emperor back in his chair.

fuck it, break out the sisters of battle, we're gonna need more blood