He is purely out for his own wealth. He is one of the most absurdly overpaid execs in the US. How anyone can become a billionaire as an employee is utterly beyond me. Taking no risk, getting all the reward.
You are thinking of all employees as fungible replaceable units, but employees are capitalistic units that can get monopoly power or economic rents.
An employee can provide a function that can’t be replaced, and that employee can capture a lot of the business profits, well beyond their actual “value” to the business. For example, Tim Cook’s personal relationship with Chinese officials might be very hard to replace.
Alternatively there is a kind of blackmail value, where losing a prominent employee can have high costs to the business, and the employee can get paid more than they are “worth” due to that. For example, perhaps Tim Cook leaving would cause more than a few billion damage in profits or valuation due to perceptions.
Again, it is important to realise too...there are probably no other CEOs that have made as much as Tim Cook. Even amongst execs, he is wildly wildly wildly overpaid. Corporate governance at AAPL is basically non-existent.
The obvious relevant example of replacing a CEO with a ham sandwich is John Sculley, which the organisation probably still has some fear about. The other relevant example is Paul Allen, a lost decade for Microsoft.