AD champ mid does not always guarantee a counter. Cass and hard poke mages that can use long distance spells or DoT can harass in relative safety while still farming against an AD.

So "fair" wasn't a good choice of word to use. There are certain classes of champions for a reason. Kha'zix single handedly made mages obsolete. Other "assassins" that would go mid like Talon etc had their draw backs. They were pure melee outside of having a medium range ability to poke but it still cost a large amount of mana. Kha'zik has long range, large mana pool, non-expensive abilities to out harass and outburst mages.