To be complete we would need every organism ever. A scientific theory is based on empirical evidence, otherwise known as facts, and it's the best we got. There is more to learn, but what we have yet to learn is the specifics of how evolution works, it is well established that evolution is the reality. If your argument were true, for any of us to accept the theory of gravity we would need to test every single square centimeter of the universe, but that doesn't stop the fact that when you drop something, it falls.
Let me put it another way, the fact is that organisms change and become other species, the theory of evolution explains how.
The fact that organisms change and become other species doesn't prove the idea that the world as we know it today is a result of evolution. To be complete, the observable facts or data would need to explain the missing links. The missing links are observable facts too, and they render strong evidence against the idea of evolution as the beginning of the world. All the evidence in the world for evolution does not outweigh or erase the missing links. "More to learn" and "the best we got" is not enough to claim evolution as "the reality" of the history of the world and its beginning.