Imo this manga doesn't deserve all the praise it gets.
The art is pretty alright, but it appears to me that the plot is a failure.
It seems like the author had set off to tell an "anti-story" in a way. To tell the story of what happened after the story itself, kind of like Yokohama Kaidashi Kikou.
The author probably wanted to tell a slow paced slice of life story, featuring nostalgia, dealing with the passing of time, grief, the return to ordinary life, contemplation, etc... This is what happens in the first chapters but quickly it seems like the author doesn't know what to say about all that anymore and ends up devolving into a typical adventure shonen story.
I was lured in by the originality of the plot but this was short-lived, it ended up being yet another battle shonen. Had the author wanted to tell a story about adventure, why not tell the story of the original adventure, especially since it seems like it's the same one with roughly the same goals and the same opponents ? They were supposed to have defeated the demon lord, and then they go right back at it and go fight demons again? The author most likely got confused and struggled to get new ideas, which also explains why the world, characters and story are generic and uninspired.
Not a native speaker, sorry if some sentences look weird.