There will always be a "left" party. Be it the Communists, the ALP, the DLP, the Democrats, the Greens, or whoever, the socialist left in the electorate will always have some to vote for. With the Greens, their longevity will be determined by how well they manage the conflicting issues of staying true to their Green credentials, but at the same time having the policy flexibility needed when managing the balance of power.
As for "someone else could preference Libs", this is tosh. As long as the ALP is more left than the Libs are, they will always get the bulk of Green preferences (unless they go nuclear or reintroduce whaling).