Buying grevious wounds into a champion that's already healing around 4000 per ability does nothing... It reduces it to 2400, but that's still alot..
Also shielding is even worse!
that means the item dealt 1600 damage per instance. We're not going to turn off healing entirely, but 1600 before you even count the stats of the item is a HUGE amount. There's no champion that's healing 4k per ability, that would be too much and we wouldn't try and solve that with an item.