I've been with the Mac for decades and I can see how difficult it is for Apple to come up with annual updates that make people give a damn. There is only so many places you can take desktop software without changing the hardware.

Stage Manager actually looks pretty good on OSX but like you say if you've been using spaces since 10.5 then its going to be difficult to adapt but to me it looks like a clever 3rd party app you might have seen around 10.3.

On the iPad? The problem is smaller windows rather than management of them. The iPhone-size windows are difficult to see on an 11" device.