A child who lives in a new world will never know how much they are missed by their loving parents in the world they left behind.
And unlike most other isekai protagonists, who at least died before being transported. Subaru was alive. There is no closure, no evidence, simply a disappearance of the greatest treasure in these parents' life. They will go to the grave with hearts weighted forever by thoughts of what their long gone child could have been.
Subaru will never come home. And the worst part is, he misses them just as much as he is missed.