I think the gp's point is they'd rather rewrite than fork. If you fork you lose the other devs esp as you diverge greatly. But you get all the baggage.

A lot of coders doing side projects do it for fun and to learn and that often means making their own architecture decisions (and mistakes) so they can practice that process and learn

Sure. I was responding to the suggestion that the key problems with GIMP are the UI and name. The image manipulation code is surely pretty mature by now.