Don't assassins technically scale just as well as a mage mid?
Think about it. The assassin (zed, talon, leblanc, akali, fizz, or kat and maybe debatable ahri) kills ONE target, the adc or apc. From there the enemy team's dps is LESS than your team even if the assassin DOESN'T attack due to now it's at LEAST a 4 v 4 if you say the assassin does no damage after the burst (which is untrue...)
Doesn't this mean it's better to assassinate one target or in 10 seconds, 2 targets that are squishy over an AOE attack on the enemy team?