My point is intuitive isn't the same for everyone.
GIMP doesn't come from a clone of PS. It has it's own history and its users are used to it as it is. Any change to be a PS clone isn't what it's existing users (and developers) want. Forks to do this have come and gone. Single window mode is all that came out of mainline GIMP to appeal to PS users. This is part of a thing with open source, it's not possible to force something on the developers. You have to fork, work hard, win people over and become the new main branch. GIMP mainline keeps winning those battles.
Edit: oh and I am totally a freak in my software background outside of British computer people my age.
Blender used to get a lot of stick about it's UI, but it's now it's doing amazingly well. It seams to be freeing 3D from Autodesk.
GIMP seams to be going through a bit of a development phase and after GTK3 move is complete, other features will get the that development. It could be interesting few years.
As for trying RISC OS, no where is especially active to be honest. Though it can be run on Raspberry Pi. The big thing it still does best is save dialogs. Just drag into a file manager window. For the decades after leaving RISC OS I have to copy paste directory paths like a primitive! The ROX Linux desktop gives you a bit of a taste of it, but only ROX apps have the dialog magic. Last I run RISC OS was ArcEmu to play Bug Hunter 1 & 2. I did some open source work on RPCEmu to run games I made as a kid. I should run it again to show the kids what I was doing at their age!