If they cut down all the trees, there will be lesser oxygen and more carbon dioxide. Carbon dioxide is harmful to our bodies and we won't exist after a few years. To prevent this from happening, we should cut down lesser trees. If we do cut down trees then we should replant some more trees.
Hundreds of thousands of animals, plants, and trees would go extinct. Not only that but certain plants only found in rainforests that we use in medicines could be wiped out so many medications would no longer be available to us
All the animals would die a slow and painful death