https://marco.org/2007/10/31/if-leopard-shipped-and-it-looked-essentially-the
If Leopard shipped and it looked essentially the same as Tiger, I think most non-programmers would pass it by. At first glance in screenshots and the back of the box, potential buyers would not see any signs of change, and therefore would not see any hope of new experiences which justify the cost. This would do a great disservice to the mountain of improvements in the underlying system.
— Satisfying UI Design is Often Illogical (thanks John)