> But for any given change the "simplest thing that can possible work" is usually something along the lines of "we'll just add another if-statement" or "we'll just add another parameter to the API call".

Sounds to me like we need to distinguish between simplicity of the individual diff, and simplicity of the end result (i.e. the overall code base after applying the diff). The former is a very one-dimensional and local way of optimization, which over time can lead you far away from a global optimum.