Suppose there is also some random variation in how big they must be to be able to split. The big and small splitters would both survive and reproduce equally well, so nothing much happens. Then, let's say, the climate changes, it gets colder, which wipes out the normal prey for a predator, say a sea newt. So the newt switches to eating the creature that reproduces by splitting, but the newt is short-sighted, so it can only see and catch the big ones. In sexual reproduction, the two half decks, shuffled, join to become the new organism.
Nothing succeeds like success.
The replicators replicate because their vehicles survive and reproduce. Individual success is based on survival and reproduction, so every creature on earth now is in this sense equally successful, because they are all still here. All replicators inside them are likewise successful. But I'm not sure whether time is the right measure for success. Any gene that codes for a trait that fails to get it successfully to the next generation is unsuccessful.