It still isn't 'going backwards' despite reverting to a previous state. The fact that a species manages to survive regardless of the path of evolution it has followed represents those genes successfully sustaining the species for a prolonged period of time. It all comes down to what conditions an area proves optimal for survival, and if that means fins are better than legs, then that's exactly what the best surviving species will eventually do

Yes but the fins are not at all the same. The function is the same, but the structure is not.