The Star Wars franchise rests entirely on the legacy of the first two films, with special emphasis on The Empire Strikes Back.
If we want to pare it right down, I think we could say the franchise rests on the conversation-fight between Luke and Vader.
Specifically
Darth Vader said:No, I am your father.
Without ESB, Star Wars would be remembered as the first of the grand space operas of the Eighties, while another one would take the place of 'timeless space opera'. We'd have a world where practical effects veered more in the space opera direction than towards horror, I think, and their Jurassic Park would be a space film.