The Void feeds on everything indiscriminately. There’s nothing it won’t consume. The Light doesn’t seem to feed on anything. The Mag’har blame the slow death of Draenor on the Draenei and the Light, but where is the proof? We’ve seen the Light used to revive dying lands, to cleanse them and restore life to them. You need only look at the Plaguelands for proof of this.
If the Light truly consumes Life, then Quel’Thalas should be a wasteland thanks to the Sunwell’s new holy property.
I think it’s more likely Draenor’s decline was either a result of the lingering effects of Fel thanks to Gul’dan and the corrupted Iron Horde, or possibly just a result of that timeline fading as all timelines, save for the main one, eventually do.