I feel like mages are sharing their runes with supports and melee champs while other classes are not

Check this out

Mages get 1 dedicated keystone and 2 shared.

-Summon Aerie is more for supports than mages -Spell slinger is more for melee champs than mages because the slow resist only works on melee -Arcane comet is the only dedicated keystone for mages.

ADCs get 3 dedicated keystones

-Press the attack -Lethal tempo -Warlock blood lust

Assassins get 3 dedicated keystones -Thunderlords/electrocute -Dark harvest -Predator

Its like the sorcery path is shared by mages, supports and melee champs while the other roles get real dedicated keystones for their role.