Oda really seems to like his natural catastrophe plots.
He waited so long until the fire got so big that almost no one's powers are strong enough to put out the fire.
I don't think that the flower capital's destruction is impossible. We had this before(Skypiea, Dressrosa, Zou etc ). Oda wants the plot to force all citizens of a society to work together to built up everything from scratch, with a focus on their dynamics and teamwork.