It's more reproducing than surviving. If the population of some species increases and the number of copies of some allele remains constant we could consider than gene less fit than the other alleles, in the population genetics sense. So it's frequency rather than survival that geneticists look at. But that proves that there are indeed other ways that they could have defined fitness if they wanted to.
Well that's a given mate, nobody's talking about populations that dont reproduce are they? And even then you're still making the same tautological mistake.... If they survived they must have been fit