Again, most of this isn't really resolved by rerouting that power to a different cosmic force. The Legion would still have had access to many of these things (vast necromantic armies and weaponized plagues -- also runeblades used to be just enchanted weapons like Felo'melorn) through the Nathrezim except for the Helm of Domination and Frostmourne (even though it's just a soul-stealing weapon of which the Legion can make infinite amounts). What it then boils down to is whether you think Shadowlands patching up the plot hole (why the Legion only ever produced one Lich King) justifies/makes up for all of the retcons, hastily stitched together storylines and new plot holes that resulted from this decision. You can't really have one without the other.