The Maw that we see South of Torghast(North of Icecrown since it is mirrored) is what the Jailer dragged in from other Death Realms after being imprisoned in Torghast.
What we see is all there is. If the Maw and Torghast merged with Azeroth and Icecrown citadel then we’d simply have the Maw crash into the Ocean north of Icecrown(drowning any Mawsworn since the Maw’s ground would be facing towards the Ocean) while Torghast merges with Icecrown Citadel.
The most evil of sinners would still be sent to the Tower which while a massive-multizone sized Fortress would still be escapable without the Jailer and cause problems for Azeroth itself.
Azeroth being pulled into the Maw is a problematic scenario.