I'm not an emacs user, but it's interesting to look at the sort of changes going into a program that was originally written in 1981 (Gosling Emacs, the basis for Stallman's Emacs).

There's not many other programs that are in a similar place of very old + still in widespread use + still seeing regular feature development.

The impacts of what sort of design decisions linger on, and for how long? What change is born out of the program itself vs. having to adapt to changes in the environment? What's more likely to result in defect counts going up again? All sorts of interesting data points re lifecycle ...