Microsoft when Balmer took over had one direction to go: down. It's reversion to the mean. Eventually a company is going to roll 7 on the craps table.
Unless Cook is extremely lucky and skilled, in 5 or 10 years people will say the same thing about Apple. Days of stagnation and decline will also visit Google and Facebook eventually.
I understand our fixation on leadership. It is super important. But as a thought experiment, I wonder if Steve Jobs was running Microsoft 2000-2010 instead of Steve Balmer, would Microsoft be in great condition today? Or were there already internal and external forces at play at that point that were trouble for Microsoft?
I guess the ultimate lesson is one you can learn from Bill Gates: quit while you're ahead.