The end reveal as to why Professor Matsumoto chose Vivy, of all AI, to fulfil the Singularity Project : She was the AI who he visited all throughout his childhood and up to his adulthood in the AI Museum, who showed him how kind and human A.I.s could really be, given the chance.
Estella and Elizabeth’s Heroic Sacrifice, sending away all of the residents and staff of the Sunrise and averting the disastrous crash from the original timeline, singing in unison to calm the humans while they burned up with the ship on impact. This act is so monumental that it does a complete 180 on Estella’s legacy, changing her from being remembered as the most malfunctioning AI in history to one that upholds the ideal to what an AI could be in the revised timeline.
Added to by the song. We have Ensemble for Polaris, the song which acts as the Ending Theme for the second arc, sung by Estella in Episode 3 to accompany her guests' breathtaking view of the stars, and by both Lifekeeper sisters to bid the guests farewell at the end of Episode 4 as the pair sacrifice themselves to prevent the Colony Drop from impacting a city below.
The growing trust between Matsumoto and Vivy as the Singularity Project proceeds.
Vivy finally figuring out how she can sing from her heart: Remembering all of the people she’s bonded with and experiences she’s had across her century long journey. It’s such a beautiful realisation that The Archive decides to let her decide whether or not to render humanity obsolete, giving her the chance to sing her own song and shut down all the A.I.s.
Matsumoto managing to restore Vivy after her Heroic Sacrifice in shutting down the rogue A.I.s and ending the war. While tempered by the fact she doesn’t have her past memories and experiences, Vivy is finally free to make people happy with her singing without being burdened by the trauma she had to endure throughout her century-long journey.
, the song which acts as the Ending Theme for the second arc, sung by Estella in Episode 3 to accompany her guests' breathtaking view of the stars, and by both Lifekeeper sisters to bid the guests farewell at the end of Episode 4 as the pair sacrifice themselves to prevent the Colony Drop from impacting a city below.