Luke, Han and Leia didn’t fail as long as Poe, Rey and Finn and their friends continue to value their sacrifice and fight for what they fought for. Like Han told Ben Leia’s beliefs still were worth fighting for even in her death.
Like the Skywalker line ended but it really didn’t since Rey has adopted the name, and she also took the mantle from Luke of restoring the Jedi order. So Luke didn’t die for nothing. She is the daughter Luke never had. His life had purpose he inspired another generation of heroes.