While Apple is often regarded as the quintessential design company a large part of their continued economic success is built on their supply chain. Tim Cook is the right person to ensure this key factor to Apple's success. It's not glamorous but for a global company like Apple it's essential.
When you start to consider the numbers, it gets quite crazy - 200 million iPhones a year need 200 million LCD/OLED panels to be made and delivered to factories in sync with production demand, same for NAND flash (Apple buys about 20% of the world's capacity for NAND production every year, and given that starting capacities are 64GB these days, Apple must be buying at least 12 Exabytes of flash a year). They also then need 200 million of their custom CPU/GPU chips to be fabbed, 200 million aluminium cases, etc. etc. Right down to 200 million little SIM ejection tools to put in the 200 million instruction booklets in the 200 million boxes.
It boggles my mind when I start to think about it all :)
And you also need to be a great salesman, which most artistic oriented people hate.
Note that by this definition, Cook isn’t a good replacement as well. He seems very much concerned by « doing good » or moral / ethical issues than jobs was (at least publicly), and he likes to invite famous people to talk about the products, but i don’t feel the same passion for this junction of tech and art that can only rise when you’re personally living in that junction.
At least that was his job at a time.
I'm sure I've seen him answer this question in interviews in the past.