[BlackFly]

Agreed; you can reject exponential decay a priori: a code base has some minimal set of functionality that it specifies and there is some minimal set of lines required to provide that functionality. If you believe this, then the "decay" curves, must illustrate an asymptote. The asymptote only gets reduced by breaking backwards compatibility. Such an action would include projects like Angular that go ahead and throw away a ton of core functionality in moving from 1.x to 2.x.

The decay isn't actually decay at all, but represents the complement of lines that define peripheral functionality. Lines defining peripheral functionality typically require modification (refactoring) as additional functionality is added. The asymptote which all the curves illustrate but the fit cannot capture represents the proportion of irreducible core functionality.

Simply adding a constant to the fit might fix all the problems.

[Nomentatus]

This is a close fit to Fechner's Law (not Weber's) relating perceived intensity to stimulus in animal vision. Also, thanks so much, author.