I'm pretty sure most of the prisoners would only be injured, not killed. Physical bodies would be destroyed.
It would only buy a little time for the mortal world... years, maybe? Even decades? Barely enough time to get a new cadre of "senior" wizards up from the mid-tiers...
It depends on
when the failsafe goes off.
The failsafe was beyond a nuclear level blast, probably more than Tunguska or Krakatoa. Anything mortal or on par with a Nagloshii would be capital "D" dead. But the scheme was for Halloween, a time when even the immortals can be killed. I believe that everything that is tied to one physical body would have perished but that still leaves a number of beings on the Winter Queen level or above that would regenerate over time like Sauron or find a new host body like the Winter Mantles.
Maybe some of those things don't hop directly on the Outsider train but enough do to overwhelm the remaining Accorded Nations(who are definitely going to splinter) and that's The End. It's just that it might not all happen immediately. The Mortal world won't be finished recovering from the event before the first Cthulu level threats start popping upand the burst of energy over those ley lines would have stirred up every dark power hidden in the Western Hemisphere to kick that off. The apocalypse might take a century or two to run it's course but it would be too late to stop it.