Most of my side projects have functional core features that I use regularly but they aren't quite shippable.
Building a GUI for others unfamiliar with the internal logic is incredibly difficult and tedious.
It could definitely happen after a few years and yes I have already had cases where I had to trace the code again to understand them. It is still a major hassle though!
I occasionally build simple CLIs that are immediately intuitive but GUIs are just too much work.