1) Just looking at the scene you can tell he knows what’s going down right then and there. He’s in awe of what’s happening, and he’s despairing:
(gifs by lordkrushnic)
2) He has not technically fallen—at least, not in the same way Anna did, or in the way these other angels have by being jettisoned out of heaven. His grace was taken from him by Metatron and put in a bottle (with all the potential of eventually being returned to him).
3) Metatron’s comments about him living a normal human life were part of him twisting the knife—because Cas will still be Cas, and to Metatron, locking him into a flawed mortal life with his memories intact is a perfect punishment for this human-loving angel.
4) This is also not another Emmanuel situation because it’s already been done, and the writers aren’t that stupid, jfc. It would literally be pointless.
5) You really don’t think the writers would pass up the chance for YET MORE ANGST for Cas, do you??? *sobs*