The narration for all the different endings changes depending on the narrator. It's not all the same dialogue.
How much explanation did you want to happen? You can't have everything spelled out for you. The story arc ends, but the universe will go on. It's a shittonne better than the previous version where nothing happened at all and you had to guess.
It's also established pretty strongly throughout the game that you couldn't win against the Reapers. That wasn't going to change.
Also why are you still so confused about the Reaper origin story? It's spelled out pretty clearly when and why. Or are you complaining that you get it and just think it's stupid?