I guess you weren't really paying attention to the Destiny 2 reveal, where Bungie stated many times that Ghaul causes the Guardians to lose their Light by caging the Traveller. You see Zavala collapse when it happens. So yes we can lose our Light (notice the capital L - it's a proper noun and is not the same as light). It's been made very clear throughout the game and grimoire that Light is the power of the Traveller (divine presence of the Sky), just as Darkness is the power of the Deep. It's the very reason why the Darkness and its servants have been trying to destroy the Traveller. So isolate the Traveller and no Light. [spoiler]Here's a little clue to how we get our Light back. There's a huge shard of the Traveller in the EDZ - a shard that is outside of the Cabal's shield.[/spoiler]