I’ve been catching up on this thread. People keep asking why Luke died.
According to the novelization, Luke gave himself over to the Force. Luke, before disappearing, even thought he heard the voice of Obi-Wan saying “Let go, Luke…”.
If you think about it, Obi-Wan and Yoda also gave themselves over to the Force in a similar fashion, especially since Obi-Wan disappeared an the moment before Vader’s saber hit him.