Trust me, the majority of tiling WMs has on workspace for ALL monitors combined. The percentage of WMs treating every monitor as a separate unit is smaller.

And sure, you can send message and commands to Niri via IPC, but that is very very different from say XMonad or StumpWM where you can actually override or alter functionality within the config instead of having to change the original source code.

That I'd say is the very definition of hackable. When you can actually just throw stuff into your config to change core behavior.