In Theravada, bodhisattva is bodhisatta in Pali and refers to a being on the way to buddha-hood. The Buddha of our time, Gotama (or Shakyamuni as he is sometimes called in Mahayana) was perfecting the paramitas over countless lifetimes, literally tens of thousands of lifetimes.
Karuna (compassion) is one of the ten paramitas in Theravada Buddhism. The Buddha was perfecting dana (generosity) and compassion in his most recent lives before being born the final time.
Becoming a Buddha or a fully enlightened arahant is no easy task and so there are countless lifetimes to perfect the paramitas and to help others. In my opinion, waiting until all beings are liberated is sort of apocalyptic thinking, such as the thinking that one day the world will end, not just the physical world, but the spiritual too, that all beings will be liberated and there won't be any more beings at all anymore. Physically it would mean that all animals, insects, reptiles, birds, fish, and sea mammals would be dead and re-born as humans who would all enter nirvana at once? In the last days, there would only be humans and no other animals of any kind? In my opinion, I just don't see that happening.
There will always be some sort of living beings in this world system or another. But we can be compassionate and do what we can to assist others. Since enlightenment is most likely a long way off, we can show compassion in any way we can along the long journey, as a Theravadin or a Mahayanist.