In some ways I'd agree, it is a tautology. But this means that it is possible to derive mathematically some general theories of evolution (e.g. the Price equation), which means that logically they have to be true! They can then generate testable biological hypotheses by making predictions, or partitioning different selection occurring at different levels in the biological hierarchy.

So I would suggest the main reason your criticism doesn't work is that fitness really has to describe a causal relationship between the trait and reproductive success. That is, we need to be able to say that "those individuals with trait X leave more offspring because of trait X". Such a relationship could be investigated experimentally. This can be contrasted with random processes, which can also affect the number of offspring an individual leaves. Though such processes may be important in evolution, this isn't evolution by natural selection.