In chapter 900 we see a canon opens fire on the Sunny and appears to destroy it. We are left with only the image of the Straw Hat’s Jolly Roger floating on the sea.
I don’t think that the Thousand Sunny is really burning down. It’s the ship built by Franky… and if Franky’s dream is to create and travel with a dream ship which would experience countless battles and reach the end of the Grand Line, I think his ship will not be destroyed.
All the dreams of the Straw Hat Pirates will come true. That’s the reason for their travel. The Thousand Sunny was built from wood hewn from the treasure tree “Adam” like the Oro Jackson. The Oro Jackson is the ship of the former Pirate King Gol D. Roger. In my Opinion, it makes no sense to burn down the ship of the future Pirate King.
But we see in the last panels from the Manga, that it looks like the Thousand Sunny is hit by the Queen Mama Chanter’s cannon fire, and sinks into the sea as the sail bearing its Jolly Roger is shown floating through the ocean-like wreckage.
Now my speculation: What if the last panels that we saw are not the current situation rather than a fake memory that Pudding’s Memo Memo no Mi gave to Big Mom with the cake or something else? Now Big Mom thinks that the Straw Hats have been burned. This will give to Luffy and his Nakamas the chance to sail away and escape with the Sunny.
*Theory by Blackleg