Krypton was amazing, a great show with the Brainiac I hoped to see on film.
But we need a Superman movie, not a Legion movie, not a Metropolis or Smallville movie. Superman along with Batman and Spider-Man are the superheroes everyone knows about, so they don't need to tell where he comes from or how he can do what he does.
If Gunn is going with a Year 2 story, he is making, imo, the right choice to just hit the ground running and tell the best story he can. I just hope he doesn't put too much of the wide DCU in the story, and keeps the focus on the Superman mythos, at least for the first movie.