I’m not sure that I agree that Vonnegut is digressing. I think it is more accurate to say that his non-linear narrative style appears to be digressive, when in fact each temporal jump is a necessary element in the story. My own take on Slaughterhouse Five is that the book is less about Dresden than it is about how one responds, psychologically, to trauma and about the uses of narrative to make sense of the terrible things that happen to us. The book is at once an attempt at escape through the imagination and a satire or commentary on the ineffectiveness of that escape. In that sense the digressions are both essential and comic. Vonnegut’s narrative style is an attempt to stretch conventional storytelling to make it possible to understand Billy’s experience, one that Vonnegut himself recognizes as inadequate. It’s telling that the alien Tralfamadorian race has developed a literary form that can capture experience—it’s just that humans can’t understand it.
The “So it goes” mantra works in a similar way. It is at once an expression of resignation, and a physical description of the plot, which will “go” regardless of reader expectations for linearity.