The advancement in stories change. What on Earth are you talking about? Stories EVOLVE and CHANGE as the STORY continues.
A story set in stone is a linear projection of a sequence of events. What happens when those events are over? Does the world end? Uh, no, it evolves, it changes, and it continues. Lore is written for a certain period of time, and then new lore has to be written. TES isn't in some sort of a time capsule that doesn't allow change. Races evolve, entire continents get overrun by other races (Vvardenfell), etc.
Learn some good storytelling, or read a book series.
You are right, but you're painting everything in the same brush. If there was indeed something in the game that explained why they are what they are, then all is well. If, however, we're just given white goblins and expected to roll with it then... not good. Yes yes, I read 'How to write a damn good novel' both book one and two!
Whilst history will change (and we're talking about history here, not stories or lore, I can give you many examples of a story that never changes... or a lore that is set in stone, you're being general and vague), and new events will unfold, you can't expect people to live with a sudden miraculous change that removes all the mystery entirely. A better 'story' would be if the Falmer were still mysterious and unknown, with the player being the first to discover more about them... eventually finding out they still exist. But there is none of that it seems... they're just common goblin-critters that we might find everywhere, possibly with no information on their current well-being.
If they suddenly made the Reguards white and the Imperials black with zero explanation, nobody would be happy. For a change to feel real, it needs a transition. Only transistors are capable of switching from 0 to 1 in the blink of an eye.