Grave of the firefly is not a cautionary tale. You are failing to put it in its cultural context. Seita mostly acts or try to act as would be expected from a Japanese man at this time and the people around him are actually not really trying or failing to help because they are trying to survive the war too.
Takahata is actually extremely sympathetic to their plight.
In a lot of ways it is a reverse Hollywood war movie. There is no real heroism. Courage is pointless, people selfish in their will to survive. The protagonists are clueless and ineffective but they are touching in their humanity and try to root for each other and yet they fail and die forgotten.
It’s really a movie about how much of a waste war is. Takahata doesn’t even try to introduce suspense and plays it straight from the beginning. You know from the beginning that the protagonists struggle is pointless and that’s the point.