I’m not sure that was Apple’s intention at all. My gut feeling is Stage Manager was designed to support and enhance iPad multitasking, and to enable iPad to offer full external display support. I think it was added to macOS to open up a consistent multitasking experience across devices. In other words, anybody coming to macOS from iPadOS would hardly see a difference. Otherwise, it makes almost no sense.
My case for this position follows….
The public has been living with pointer-and-keyboard gui interfaces for some 38 years. I don’t think most users were having a hard enough time with macOS’s windowed environment to justify Stage Manager to solve a problem for the few people who struggle that much with window management.
Also, using it day-in-and-day-out I don’t find it to be an easier form of a multi-windowed interface. I think it’s different and that’s it. One may have a preference in terms of which form appeals to them, but one is not inherently easier than another. With one exception. It is a little bit harder, but not impossible, to get lost with a messy desktop while using Stage Manager. But with Stage Manager you could get lost with a messy set of stages (and full screen windows in other spaces!).
In other words, I think these are really six-of-one-half-dozen-of-the-other interface options. As such, it seems Stage Manager on macOS was not to solve a user interface problem on macOS, but to solve the problem of bringing parity of design between multitasking devices (and, to take it one step further, was done because it was easier to conform the mac desktop to the iPad experience than the reverse).
Q.E.D. (Maybe?)