I recently heard someone make the argument that, from what’s implied in dialogue in ANH, the Empire shouldn’t be able to maintain its control of the galaxy without either the Senate or the Death Star. A lot of Tarkin’s dialogue points to this idea that the Empire needs the fear of the Death Star to keep control of its subject systems. With that in mind, and given how ANH ends, if ESB had opened by telling us “The Empire is falling apart after the loss of the Death Star,” that would have been an equally valid path for the story to take.