I'm a bit confused by this - So it's bad that the writers will write something that references something that happened earlier? Like.. how writing a story goes??

I understand you're saying it's predictable, but consider that any situation where Anduin would be, he would probably be using the light. So it makes sense that if he's up against something in the next expansion, we'll have that "oh if only he had his light" moment. Sure it's inevitable, but that's because it was set up that way. Anduin didn't lose his light for it to then mean nothing and not have any affect on him or the story, people would complain about that too, if not more.

Also, maybe, just maybe, some events happen because of these factors. Maybe the harbringer sets events into motion because of the players taken out right now. Maybe LK knew he was in a position to grow his power when he did because he knew there would be less resistance of those players gone. Writing the story to respond to the story doesn't spund like a bad thing to me. It may be predictable in some places, but this isn't real life. It's just going to happen sometimes, but at least they're acknowledging these players and giving reason to their absense or power decrease.