My hunch is that if there were more unicorns (amazing designers/illustrators who used GIMP and had the capacity to write code for it), you'd have more progress

My hunch is that if there were more unicorns (amazing designers/illustrators who used GIMP and had the capacity to write code for it), you'd have more progress in this realm. For example, Figma (non-open source) is doing stuff that myself as a designer, I find very subtly innovative for a browser-based graphics program. Sure, they are actual company which can devote more resources to the product, but I think it helps that some very talented designers are using it and thus, can give feedback and suggest new features with engineers to support the effort. Similarly, I'm sure a lot of open source projects have thrived because the contributors are active users whose skillsets (coding) overlap with the project.