Pretty sure Mages aren't supposed to have a keystone.
The whole point of having different trees in the first place is so that 1 tree excels in an area that others don't. The sorcery tree is a treasure box of passives that mid laners love so naturally it'd sacrifice reliable/high damage for its utility purposes. What is the point of separate trees if you can get something as good as electrocute without sacrificing the amazing runes you get from sorcery like Mana Band, Ultimate hat, Transcendence, Scorch, ect.