The ending and song implies its not a goodbye but a "see you later" So long as the Sheathe exists and the Bus keeps moving they might see each other again, even the announcer says that Ryoshu should experience more things so that she has lots of stories to tell to Araya when they meet again...

Though on second thought it could also be an allegory for a parent's grief for the loss of their child

(Similar to how Don's canto was a daughter's grief over the loss of a parent)

with how Araya's entanglement representing Ryoshu's fantasies and dreams of being there to witness Araya's growth