I can't speak for the parent but I would say the most commonly seen effect of complexity is stagnation. Change becomes harder, promised features take forever, the developers lose motivation and significant new releases grind to a halt.
Atom exists because ST3 was seen to stagnate, ST3 exists because TextMate was seen to stagnate etc. They start as simple but incomplete tools that are moving fast and blah blah - its just the software circle of life.
TextMate 2 actually tried to solve the stagnation problem by open-sourcing. It has worked quite well and updates became much more frequent. Still though, it doesn't feel as easy to extend as Atom or VIM.
TextMate 2 is, thankfully. Although the point still stands; one editor I'd love to see take off but doubt is Vico, a modern "Vim-esque" editor written in Nu :)
Atom exists because ST3 was seen to stagnate, ST3 exists because TextMate was seen to stagnate etc. They start as simple but incomplete tools that are moving fast and blah blah - its just the software circle of life.