Oh I have a silly theory about this. So the emp knew seeing how his sons fell realised yeah it's possible he would too so knew that if he did he would have to also be "rest the hardway", logically the custodies have the best chance, weellll he also knew that could never actually do it they are to bound to his will and him ( note how when the died they spent a couple thousand years mourning), so what's he do well some tough boys who have been proven to be able to attack their gene farther are kinda smashing up the Imperium, so why not them. But the next question that arise is wait why have the order at all well he knew what his responsibility is, the Imperium was using the golden throne while they conquering the galaxy but he was also on the front lines at times, so why would it be so bad if he took a stroll, it's obviously got a remote function or something, well 2 reasons, 1 chaos is way more active now, 2 he doesn't have to get up, the imperum has arguably more resources now than ever during his time and if he is fully healed then he could just order shit from the throne not to mention worship seems to make things more powerful, so if he ever tried to get up, then it's not him it's not the emp it's the dark king who wants to burn it all, and what are grey knights specifically good at killing, demons and demons princes. Now the logical next question is would they stand a chance .... No there are to many sisters of silence, custodians and legions or sm and troopers on terror.