Do you think these creatures are considered equivalents from Light and Void? Are they equal in strength? I know that Illidan easily killed Xe'Ra, but what if the Naaru and the Old Gods work differently? Maiev once said that A'dal can destroy cities and mountains. Illidan described Xe'ra as much more powerful than A'dal (can it destroy continents?). What if Naaru has tremendous destructive power, but at the same time they themselves are very vulnerable? And the Old Gods on the contrary. They are very difficult to destroy, they are very durable, but they cannot cause the same destruction as the Original Naaru (I mean destruction by the void magic, physically the Old Gods the size of a city and a mountain).
P.S. if the Prime Naaru is an analogue of the Old God, then usual Naaru (like O'ros and M'uru) can be the analogue of the Forgotten Ones(mini-Old Gods like Iso'rath or Shu'ma(https://wow.gamepedia.com/Forgotten_one)).