You're assuming every fight will be concluded before we get back to the rooftop but we're not even sure if all the fights are already set up, there's still Sanji, Yamato, Marco, King and Queen lacking a sort of "final" scenario as well as other plot threads like the scabbards. Even up in the rooftop we're not even sure if this will be the final setup for the final fight of the arc, Oda could come back after a few chapters and show the supernovas struggling.