Nothing is ever feature complete, or alternatively, and image editor is feature complete when it reads my mind and spits out the perfect image without me doing any work. Anything short of that is room for improvement. Sure, there are a lot of people who don't need everything photoshop offers, but there are also a lot that do so people will continue paying for proprietary software for as long as that proprietary software gets the closest to spitting out the right image with the least effort.
Photopea is a feature-complete online image editor. It's widely successful. It's not open source, but it's the work of just one guy.