Its not that its even a terrible idea; its just that it seems half-assed.

Take the interaction gestures: You can slide in from the left to see the spaces but also still use the normal slide from bottom too: why repeat this interaction?

On the desktop you might want to slide in from the left to bring up the spaces but instead you get the increasingly-more-useless widget view so you still have to use the bottom gesture. Why not get rid of the widget view altogether?

The windows are still too small as well. If I have one app 2/3 and one app 1/3 of the screen I still get more real estate using the old method.

If Apple were serious about Stage Manager as an interface they would have eliminated the repeat and dead interactions but they didn't. And why? Because Stage Manager seems less like a multitasking revolution and more like a 'so, has anybody got any ideas how we can do something with iPad OS 16?' from a board meeting.