I couldn’t understand how the fact that the Overview is limited to one-dimensional stacking and the fact that we can still create virtual desktop with the mouse are related. Something like Temporary Virtual Desktop is just a way to speed up virtual desktop management by taking for granted that if you try to move a window to a desktop that doesn’t exist yet such desktop must be created, and by removing empty unused virtual desktops.
I agree that the Overview is somewhat limiting the layouts for the virtual desktop grid, but we still have the grid, even accessible with gestures: if we could make the grid auto-arrange in so that it fits the screen better, like automatically staking desktops 2x2, 3x2, 3x3, 4x3, 4x4… the grid would come handy again; the Overview could then just flatten the virtual desktop array in a single row.
But I understand that something like Temporary Virtual Desktop would mess with people workflows, if they are used to having a fixed grid of desktops: they would need to open enough windows to automatically create enough virtual desktops to recreate the same grid every time.