What happens is that as the population becomes fitter is that the ratio of favorable to unfavorable variations declines because the possible improvements are, so to speak, used up. The difficulty with the Darwinian thesis is that it doesn't very well account for functional change. The standard tactic here is to invoke exaption, i.e., to claim that a feature already of use turns out to have a different use which may be improved in turn. Consider again those pesky birds. This in nowise establishes that these putative feathers were at all usable or relevant for flight.
Ratites have feathers which are quite good for insulation and utterly useless for flight.
We then are told various tales. One tale has it that there were tree dwelling dinosaurs who took up gliding. Another has it that feathered arm motions in ground dwelling dinosaurs aided them to move more quickly. It is not necessary in these stories to embellish them with substance. Thus it is not necessary to whether there were genetic changes that could have transformed insulation feathers into flight feathers.