This new "Calamity" could arrive not immediatly after we defeat the Empire or Elidibus, or whatever battle serves as a catalyst. It could be like the Meteor, where the players defeat Nael, and are just told that it didn't work and Dalamud is still falling. Maybe we think we won in 4.5, but something wrong starts happening that WoL doesn't notice, and in 4.55 we see the full consequences of that, culminating in some end of the world scenario.

Then, maybe, we get told that we have to go back to the past, and the patch ends with us going back to fix stuff, but then are told to wait a bit more in the past before we start changing stuff. I mean, they can't just erase the world and leave us with no world to play in while we wait for Shadowbringers. It could be just as before Heavensward, when we are told to just wait with Haurchefant a little until we get permission to enter Ishgard.