The linux kernel does not break its users, save rare exceptions (security fallout).
I've rarely seen a program maintain two UIs forever when they feel like refreshing their looks
Not that I mind UI change, but I think the comparison misses the point: if it's good enough, for some people UI breaks just cost more than they gain in the redesign. So it's not about reaching perfection. It's about finding a UI only just solid enough that it can stop breaking.
I don't necessarily agree personally, but I can understand that point of view.
