What happens if there isn't any land left to buy?

After you have opened all 4 islands (Orchid, Sunflower, Laurel, and Willow), then you wait for the a 5th island, but no one knows how long that will be. To continue progressing in the game, you'll need to continue to add more Flower value to your island, so you'll have to replace low-flower decoration with high-flower decoration. The higher the flower value, the further in the game you will go.