I don't see the point of the change. It looks too much like iOS.
Stage manager definitely gives me that vibes. Apple trying very hard to make the UX feel same in a laptop and iPad.
The new one is not perfect. For example, the on/off switches are ugly and the layout is far from great, and I like “preferences” better than “settings”, which makes more sense for an appliance than for something you adapt to your liking. But the old one was very much not “not broken”.
I've never been able to find what I needed quickly in the old UI, always felt disorganized and random. I like the one on the iPad a lot better, I find it way easier to navigate, so I'm happy for the change.
So yes, it functioned poorly, while also looking terrible by today's standards.