While I can't call this a great ending, or an expected one, in retrospect I think it is a, let's say "correct" ending. It doesn't explain everything about everything, it lets you wonder, it lets you think and figure it out for yourself.
KingAkiraTRK said:
the main theme was about people's fear and how they dealt with it when the world fell apart, and to that it did a good job until the very end.
I fully agree with this. The manga focused on the chaos, fear, psychological nature of such early-apocalyptic scenarios and left us as hopeless as the characters themselves. The sad truth is that no answer is exactly the answer the manga seems to try and represent, since it is from the perspective of the victims on the inside.
Could Japan be the only place where this happened? Could this have been caused by someone, or something? It doesn't matter. The only truth left in this world is ash and fear, and maybe there is no reason or need from even the characters themselves to look for more answers as the reality of the end is, in a way, a revelation.
You could say that the ones who figured out the truth were the ones who surrendered themselves to the fear first.