...When someone goes to Hell, what stops them from getting out? Do people go to Hell even without any relationship whatsoever with an individual devil? What brings them there?
...That would imply that at least some people have a link to Hell itself, right? And thus it could be a node in the propagation.
And even if Majustis is an instantaneous effect, there are still likely ways to (a) figure out John is the center of the effect, and (b) scry John's location in the past when the even occurred.
I believe Hell is in the Sphere of the Gods, and thus is outside of the multiple timelines making up the various DC earths, so devils are 52+ times as large as nodes for the effect's jump than they would be if there were one per universe.
But yeah, this is definitely economic collapse territory. As the omake pointed out, it's not just a massive drop in the supply of souls, but a huge fraction of loans are likely now underwater because of vanishing collateral. Everyone is going to try to call in all of their outstanding loans before the effects ripple through the system, and the devils were probably already cheating on that. Even if you weren't hit by the effect, all your outstanding loans being called in could drive you bankrupt anyway.
There is a bit of a question as to what exactly constitutes one of these connections or claims on his soul. For example, suppose you had a derived financial instrument like "1/10,000th of the souls of 10,000 mortal magic-users" as a diversified asset. Would owning that instrument qualify you to get jumped to? What about a future related to that instrument? An ETF?
And that would be exposing you to one jump. If there are two, it gets way more viral.
And what if a devil gets hit, and that devil acts as a guarantor for a contract between two other devils? Or does it jump to the guarantors of all of their contracts?
In that case where three devils each owned his soul, if they went to a major hell contract arbitration firm, would the arbitration firm be one jump, hitting all of their clients with two?
(Libertarian theory predicts arbitration firms would arise in an anarchic environment, and dispute arbitration was a major thing for e.g. medieval sovereigns afaik.)