The reason there must always be a Lich King, is because there are a lot of Scourge.
Arthas raised the dead of virtually the entire city of Lordaeron and its surrounding areas, of Andorhal, Darrowshire, and all the little villages and towns throughout what are now known as the Plaguelands and all the way through to Silvermoon during the Scourge’s advance.
Not to mention all the undead Nerubians, frost trolls, etc. etc.
The Scourge were always built up as this virtually unending horde of undead, most of whom are mindless and would simply wander attacking anything and everything living in their path. Spreading corruption and killing the land as they go.
There are also more intelligent undead, Necromancers, Death Knights, Liches, Banshees and the such that are controlled through the Helm of Domination that, left to their own devices would carve out their own undead kingdoms and seek to expand and create even more undead to prolong their own power/eternal undeath.
Arthas had the numbers and the power to wipe out all life on Azeroth the entire time. However, what he wanted was an elite corps of Generals to replace his treacherous Death Knights, and so he attacked the Horde and Alliance, then “retreated” to Northrend and allowed the Horde and Alliance to fight their way through to him. Only the most skilled, most powerful of Heroes would make it that far, and the plan was to kill them, turn them undead, and use them to lead the Scourge out amongst the world.
Then Tyrion remembered he had a CC break and broke Frostmourne.
However, the Scourge remained, and so Bolvar took up the helm in order to hold the majority of the Scourge in Northrend, and keep the Scourge members who would otherwise have caused untold destruction and chaos amongst the world paralyzed while Tyrion reforged the Argent Crusade and sought about purging as much of the undead as possible. Healing the plaguelands, etc. etc.
The trouble is… the Argent Crusade is really small, and the Horde and Alliance have not been focused on killing off the Scourge either. The Argent Crusade has also spent several years fighting the risen Scarlet Crusade who were risen into undeath outside of the Lich King’s control by Dread Lord infiltrators(Balnazzar and possibly even Mal’Ganis) of the Burning Legion, so they haven’t been able to do much more than dent the Scourge’s numbers.
With the Helm of Dominaton gone, all those hordes of undead are free to roam Azeroth. All those Death Knights not loyal to the Ebon Blade, all those banshees, and necromancers, and liches are free to follow their own schemes. Even the Cult of the Damned may make a resurgence since we know it has remained active and undetected in both the Horde and Alliance since Arthas’ defeat.
Heck, a single Cult of the Damned Necromancer was enough to wipe out the entire Garrison of Admiral Taylor during Warlords of Draenor.
Without Bolvar telling the Cult of the Damned to not murder everyone… well… They make SI:7, and whoever the Horde’s intelligence service is look like amateurs.