Fitness is more than just survival.
The fittest are those that survive. Fitness' to Darwin meant not those that survive, but those that could be expected to survive because of their adaptations and functional efficiency, when compared to others in the population. The phrase 'survival of the fittest' was not even Darwin's. It was urged on him by Wallace, the codiscoverer of natural selection, who hated 'natural selection' because he thought it implied that something was doing the selecting. Darwin coined the term 'natural selection' because had made an analogy with 'artificial selection' as done by breeders, an analogy Wallace hadn't made when he developed his version of the theory.