The problems with GIMP are the same as Musescore before it was bought out - Having dedicated designers/programmers allows more sweeping and coherent changes to be made.

Voluntary fixes are necessarily smaller in scope and suboptimal as a result, leading to more complexity when those systems need to be changed later.

I'm also reminded of Casey Muratori's talk about software architecture being a reflection of the organisation that made it. Open source projects are at the extreme end where contributors and org charts are highly fluid, and communication between contributors is low-bandwidth, accelerating the complexity increase.