The question is what are you going to do if you want to launch an app relevant to the current workspace (let's say Gimp), but it's not important enough to be visible with the other windows in that workspace (editor, terminal, browser) or it doesn't fit.

Overflowing Gimp onto another workspace because of this is the main cause of workspace spam that a scrolling WM solves.

If you're working on more than one project (so, workspace 1 2 and 3 are fully utilized), where does this overflow workspace even live? And what if each workspace also needs its own Gimp instance?

There are certainly solutions to this in every WM, but the scrolling WM solution is a really simple one that never makes you ask the question of where to put something: workspaces have an offscreen overflow area.

Another way of thinking about it is: sway would be instantly improved for me if it also had a per-workspace overflow area, like maybe if every workspace had its own scratchpad that I could tile windows on, and I could reach it by moving focus into it kind like moving between monitors.