Five years ago today, Mr Tim Cook became Apple's chief executive officer (CEO), taking over from the late Steve Jobs, who died six weeks later.

Then, naysayers were predicting the decline of Apple, pointing out that the Cupertino tech giant could not possibly continue to thrive without its charismatic and visionary co-founder.

But under Mr Cook, Apple has doubled its revenue from US$108 billion (about S$146 billion) in 2011 to US$231 billion last year.

But there was no doubt that the 55-year-old Alabama native had big shoes to fill when he took over. I mean, how could anyone really succeed Mr Steve Jobs?

I was in the audience during the keynote address of Apple's Worldwide Developers Conference in 2012 when Mr Cook took the stage for the first time as CEO.

The difference between Mr Jobs and Mr Cook was immediately apparent. While the former would dominate the keynote, the latter was more than happy to let his lieutenants take over the show and let the products speak for themselves.

Under Mr Cook, Apple has implemented an employee charity programme, joined the Fair Labour Association for better working conditions and granted dividends to shareholders.

He advocated hiring diversity and equal pay for employees, regardless of gender or race or sexual orientation.

He also hired Ms Lisa Jackson, formerly administrator of the United States Environmental Protection Agency, to oversee Apple's efforts in minimising its impact on the environment through renewable energy and using greener materials.

Early this year, Mr Cook took on the Federal Bureau of Investigation (FBI) when he opposed a court order to help police break into an iPhone 5c used by one of the San Bernardino shooters.

The FBI subsequently used another company to do the job. But in defending the privacy of all iPhone users, Mr Cook gained plenty of brownie points among privacy advocacy groups.

In terms of new gadgets, Mr Cook has expanded Apple's product line by introducing the Apple Watch, iPad Pros and the 12-inch Retina MacBook. Probably the most questionable move under Mr Cook's reign was the acquisition of Beats for US$3 billion in 2014. Many felt that Apple had overpaid.

Mr Cook's Apple has followed a product-release schedule that ticks with Swiss clockwork precision, with the predictable upgrades to the iPhone and iPad.

Thus, in the eyes of many industry watchers and analysts, Apple is not innovating anymore.

But, in my book, Mr Cook gets an A grade. The profits that Apple is still making will make many Silicon Valley investors salivate.

Plus, it is unrealistic to expect innovative products every year. It took Mr Jobs six years to launch iPhone after the first iPod.

Sure, the Apple Watch has not been the smash hit investors or analysts wanted.

But the first iPhone wasn't a smash hit either. Look at how the iPhone has transformed Apple.

The next five years of Mr Cook's reign will be a testing time. Global smartphone sales growth is slowing, and the emergence of affordable premium smartphones from China phone makers is chipping away at Apple's profits.

Everyone wants to launch an iPhone or MacBook killer. And Samsung has just released an outstanding smartphone in its latest Galaxy Note7 that comes with iris scanning and that is weather-resistant.

All eyes will be on the upcoming iPhone 7 and Apple's next moves. Will Apple widen its repertoire by venturing into, say, self-driving cars or virtual reality?

Given Apple's stellar track record under Mr Cook, the folly would be to underestimate the Apple CEO.