Or maybe a reincarnation ending?
I'm guessing Tanjiro is immortal and everyone is either very old or dead. The story takes place Edo around 1920s, so baring WW2, they might still be alive... But very very old.
How is tanjirou immortal tho
He still has Muzan's blood and he conquered the sun. He is most probably half-demon since his only one eye changed back. And quite possibly Yushiro, who is a full demon, are the only demons left.
That was a skyline at the end there... future decendants?