At that point, you want just to have MDI = every image as own window and tools in own window.
no, at that point YOU want to have all those as different windows on your screen(s). Not everyone uses virtual desktops or has access to multiple monitors, and even if they did not everyone shares the same mental model. Lots of people see no need whatsoever to have a tool palette be a top-level window (from the window manager's point of view) because they view their WM's window switcher as an APPLICATION switcher, and why the hell would they want to alt-tab to a tool palette but keep hidden the image windows it applies to?
I have nothing against MDI. It obviously works fine for some people. But guess what? SDI works fine for some people too, so why shouldn't the GIMP devs give their users the ability to choose between the two modes? They are, and that's great, and I applaud them for it. What's not great is people like you condescendingly asking if other folks have never heard of multiple monitors or thinking that the stupid unwashed masses would see the light if only they'd use the proper window manager.