I know there is speculations about it beeing Genn who broke it, but what if the decay start way earlier already.
my theory is, that the distruption already started whit the opening of the dark portal first time when Medivh did it.
my theory is that, shadowlands the part we se, was actulay only 4 planets to begin whit, Azeroth, Nathreza, and 2 unkown to us planets.
But shadowlands works in a wierd way, when the darkportal was opened, Draenor became connected to, or atleast the orcs that came through, becuse they ended up on Azeroth and that for became part of our Shadowlands.
But this was not yet enough to brake the Arbiter becuse there was room.
second decay came when the Nazherim and the burning legion, came to Azeroth, this made a huge influx again, due to the Dranei becomming part of our Shadowlands again.
this still somewhat worked, but it started to become overflowed.
third came whit Garrosh opening an gateway to an alternate Draenor again, this confused the arbiter even more, due to 2 draenor beeing part of the wrong shadowlands now. and it was close to overwhelming now.
then in legion the compleate shut down came, when we literaly started to use portals to other worlds, that in now way should have been in our shadowland, and then combine this whit, Illidan opening a gateway to Argus, what most likely opened even more planets to wrong shadowlands, and due to this the Arbiter just shut down totaly.
this would meen everything what has happend is connected directly to the Arbiter braking down and the jailer getting way to many souls way to fast.