But given that that ulti has a team-wipe potential and is primarily used in the aftermath of an encounter, such as at the end of a teamfight when everyone's precious escapes and damage negation are more or less already burned anyway, or as a global zoning tool to force people to pull out their damage negation or go back to base (unless they have supreme regeneration/lifesteal of some sort), I still think it needs substantial CD or some sort of heavier costs to go with.
But idk, Karthus is almost never played. He does need help, not sure where, but I think ulti itself isn't really the top priority of receiving loves imo.