> Astounding to me that automatic rearranging would have been considered desirable

Funny, i really like this because it lets me switch to another app and then return with a single swipe. The ipad has this behaviour too though the swipe is in the opposite direction.

I use full screen very heavily and rarely use spaces as separate desktops, which I find weird. You probably consider my use weird.

Not to say one approach is better than the other, just suggesting whoever set the default might have decided based on their own use. I suspect this issue was not subjected to any UX research, even at a place as big as apple.