I think Cosmic has the right idea with a workspace-level Floating/Tiling switch.
If you try to use niri as a mainly-floating environment, you'll face lots of awkwardness: no edge half tilling, maximized windows are forced below floating, can't see all windows in the Overview. These are design choices that work well for mainly-tiling and not for mainly-floating.
Making whole workspaces floating lets you use floating-specific UX just on those workspaces. GNOME-style Overview, etc.
1
