Why does Morellonomicon only apply grevious wounds under 35%

This makes no fucking sense. As an AP champion to counter someone with an insane amount of healing, you need to get them under 35% which is hard to do because of their insane amount of healing. This just isn't adding up at all. Meanwhile we have executioner's calling which costs 800 gold and can be applied with auto attacks. It just doesn't make any sense that to counter sustain as an AP champion you need to get your enemy low while they are healing back up... WTF