In the real world you can't (and shouldn't) always say no to evolving and added functionality, nor can you always make unanticipated changes in the cleanest way given project deadlines.
The real solution is to recognize when new features are slower and messier to implement than they should be (because the evolving requirements have outgrown your original design), and periodically take the time to refactor to clean things up.
The real solution is to recognize when new features are slower and messier to implement than they should be (because the evolving requirements have outgrown your original design), and periodically take the time to refactor to clean things up.