At the end of Assassin's Creed 3, Juno took control of mankind and avoided the end of the world. Desmond is dead.

No, Juno was just released at that time, and his strength was not enough. The fourth generation should have been possessed by modern saints, but it was hit by the unknown protagonist of the fourth generation, parasitic on him, and restored its strength. Maybe Ubisoft will fill this loophole in the future, but Ubisoft has no plans yet. As for the fourth generation, Edward's memory was read because Desmond's body was found by the Templar after his death, and the fourth generation contained Desmond's autopsy report. Then they stored Desmond's genes in the form of cloud computing, so that people who had nothing to do with Desmond's ancestry could also experience the memory of Desmond's ancestors. So the company where the four generations of modern protagonists are located decided to develop a game based on this. They want to subtly influence people's understanding of the past history through games, blacken the assassin organization, and then beautify the Templars.