What would happened if koala became extinct?

It's very difficult to answer such question because we cannot say anything. In coming generations, centuries, people would definitely forget about these animals. Teachers would struggle to tell people about these animals.

The koala is the sentinel animal for bushland-dwelling species in Australia: basically, if the koala becomes extinct, whatever is responsible for its extinction will cause the demise of many other fauna and flora as well.

it all has to do with food chains and it will interrupt the ecosystem that it lives in.
the world would be horrible