As a previous user commented "the Freljord event didn't have a conclusion because the conflict between Ashe, Liss and Sej is still ongoing" i agree.
I dont understand why people are complaining about having lore in League of Legends. I do not see how i can be connected with a champion in any way if they have no background. (i understand you are not complaining about lores, but many of the others who have commented seem to be against it) You cant release a champion and just say "oh here is someone new you can play now have fun." That seems so bland and weak on an art perspective.
Now to the main topic. The justification for the lore change is the release of Azir, and i believe they are doing a good job at it as the story seems much more interesting than the previous ones. Making a more interesting lore makes the game more interesting, as you are being immersed into a new universe, in a gaming perspective, that is very important. That is like playing Mass Effect without the storyline, instead you are just shooting aliens. Some people need to be immersed into another universe and feel connected to another champion in order to have even more fun. Some may not have the need for a storyline and just want to simply play, but Riot is filling the need of those who want a good story.
I trust Riot to make this lore rework work, they are an evolving company that with time keeps improving.