You are probably right. But I'm also pretty sure that Apple cares about its culture much more than anything else. So promotion will HAVE to come from within. And it does not have to be a financial guy. Jobs was not a finance guy. I'm not sure how many tech companies are lead by finance guys.
But as you say, Cook is doing a great job. It will be his job for as long as he wants it or until health reasons create a problem. And I understand that he is a healthy living guy. And he loves the company and work. So we could be looking at 15 more years of his leadership.
Jobs wasn't a finance guy, but he was the founder. Founders are completely different than CEOs after. You see a far larger range of types in the founding CEO position than in the CEOs promoted to the position. People like Jobs would never be CEO if they didn't found the company. Same could be said about many other founders, like Zuckerberg for example.
In most cases, the CFO becomes CEO. If the company is struggling with image, it may be the CMO. If it's having operations issues, the COO. Really dependent on what it needs at the time but those are generally the most common people who get the CEO promotion (or are tapped to be CEO at another company).